TST: Implement maximally-specific/strict fixtures for arithmetic tests
gets rid of all remaining "FIXME" comments in tests/arithmetic.

Should go after #23681 and #23642 (will need to be rebased)
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/conftest.py`](https://github.com/jbrockmendel/pandas/blob/86f38103dff5d7d9d70697239ef68c4257795e8d/pandas/tests/arithmetic/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_datetime64.py`](https://github.com/jbrockmendel/pandas/blob/86f38103dff5d7d9d70697239ef68c4257795e8d/pandas/tests/arithmetic/test_datetime64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_period.py`](https://github.com/jbrockmendel/pandas/blob/86f38103dff5d7d9d70697239ef68c4257795e8d/pandas/tests/arithmetic/test_period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_timedelta64.py`](https://github.com/jbrockmendel/pandas/blob/86f38103dff5d7d9d70697239ef68c4257795e8d/pandas/tests/arithmetic/test_timedelta64.py) !

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/jbrockmendel/pandas/blob/86f38103dff5d7d9d70697239ef68c4257795e8d/pandas/util/testing.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23734?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@2c25bd1`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23734/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23734?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23734   +/-   ##
=========================================
  Coverage          ?   92.24%           
=========================================
  Files             ?      161           
  Lines             ?    51403           
  Branches          ?        0           
=========================================
  Hits              ?    47417           
  Misses            ?     3986           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <75%> (?)` | |
| #single | `42.3% <10%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23734?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/23734/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.68% <75%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23734?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/23734?src=pr&el=footer). Last update [2c25bd1...9c07638](https://codecov.io/gh/pandas-dev/pandas/pull/23734?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Travis failures are the segfault and a Hypothesis can you rebase Closing until consensus is reached on #23730. > Closing until consensus is reached on #23730.

@jbrockmendel How does this depend on that PR? I thought this PR looked OK to almost merge? (some minor comment of Jeff) >  How does this depend on that PR?

This PR papers over a problem which that PR solves.  (though granted, that one is going nowhere fast) Ah, because here also the transpose versions of the fixtures are included?