REF/TST: collect reduction tests
Similar to how to have tests.arithmetic for tests where we expect EA/Index/Series/DataFrame behavior to be analogous, this starts collecting the tests for reduction operations.  This is motivated by #24293.

This does not yet collect tests from tests.frame.test_analytics or any of tests.arrays or tests.indexes.

Optionally we may want parts of test_nanops to be part of test_reductions, since those functions will also be in the check-these-are-equivalent pile.

Parametrizes some tests in test_base.

Moved tests are unchanged, except for non-context usages of `pytest.raises` are context-ified; some tests are parametrized.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=h1) Report
> Merging [#24367](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6cf7d9e2606e82fd3934e1dac8119c43f5ee643?src=pr&el=desc) will **decrease** coverage by `49.3%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24367/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #24367       +/-   ##
===========================================
- Coverage   92.29%   42.98%   -49.31%     
===========================================
  Files         162      162               
  Lines       51832    51832               
===========================================
- Hits        47839    22281    -25558     
- Misses       3993    29551    +25558
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `42.98% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-98.65%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-95.46%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.17%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.15%)` | :arrow_down: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `10.44% <0%> (-89.56%)` | :arrow_down: |
| ... and [120 more](https://codecov.io/gh/pandas-dev/pandas/pull/24367/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=footer). Last update [f6cf7d9...6c98821](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=h1) Report
> Merging [#24367](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8c58817bd0eaca46d7c0d0c975715ad37fe43710?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24367/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24367   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         162      162           
  Lines       51875    51875           
=======================================
  Hits        47883    47883           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=footer). Last update [8c58817...e3a6fc6](https://codecov.io/gh/pandas-dev/pandas/pull/24367?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thoughts on using markers instead of (or in addition to) moving?  > Thoughts on using markers instead of (or in addition to) moving?

The idea is to parametrize in order to make sure we test that behavior matches across the classes.  I don't see how using a marker instead gets us closer to that. Hello @jbrockmendel! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/reductions/__init__.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/reductions/__init__.py) !

 - There are no PEP8 issues in the file [`pandas/tests/reductions/test_reductions.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/reductions/test_reductions.py) !

 - There are no PEP8 issues in the file [`pandas/tests/reductions/test_stat_reductions.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/reductions/test_stat_reductions.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_analytics.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/series/test_analytics.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_datetime_values.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/series/test_datetime_values.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_operators.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/series/test_operators.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_timeseries.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/series/test_timeseries.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_base.py`](https://github.com/jbrockmendel/pandas/blob/e3a6fc690b1e7da26cbbf563d69cf1df7b29d93e/pandas/tests/test_base.py) !

 ok, would like to remove the classes and rather have separate files for each. can leave that for a separate pass.