TST/REF: collect DataFrame reduction tests
NB: this retains all extant fixture usage.

Collect tests by method being tested.  Collect all calls to assert_stat_op_api in one test (one test which can be usefully parametrized/fixturized in a follow-up), removing a ton of noise from other test function signatures.  As a result, it becomes much easier to see places where we can _usefully_ parametrize/fixturize tests in this module.  e.g. test_min and test_max are nearly identical.  ditto test_cumsum/test_cumprod/test_cummin/test_cummax.  ditto test_idxmin/test_idxmax...
Hello @jbrockmendel! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 03, 2019 at 00:07 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=h1) Report
> Merging [#24914](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5761e359b65631db491349a65fc21d0da51dcc0f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24914/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24914      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52404    52404              
==========================================
- Hits        48413    48412       -1     
- Misses       3991     3992       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24914/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24914?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/24914?src=pr&el=footer). Last update [5761e35...4209754](https://codecov.io/gh/pandas-dev/pandas/pull/24914?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/24914?src=pr&el=h1) Report
> Merging [#24914](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f75a220ff1e5e027ef2b070430fd7f4490cdcbf0?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/24914/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24914   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52415    52415           
=======================================
  Hits        48418    48418           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <ø> (ø)` | :arrow_up: |
| #single | `42.87% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24914?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/24914?src=pr&el=footer). Last update [f75a220...d646dad](https://codecov.io/gh/pandas-dev/pandas/pull/24914?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 looks fine, can you merge master. ping on green. Ping thanks @jbrockmendel 