TST: parametrize and de-duplicate arithmetic tests
This is likely to be a sisyphean task...
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_datetime64.py`](https://github.com/jbrockmendel/pandas/blob/33ff3f756bbcd90c2ce775b12129c7492dd7770b/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/33ff3f756bbcd90c2ce775b12129c7492dd7770b/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/33ff3f756bbcd90c2ce775b12129c7492dd7770b/pandas/tests/arithmetic/test_timedelta64.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23240?src=pr&el=h1) Report
> Merging [#23240](https://codecov.io/gh/pandas-dev/pandas/pull/23240?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5a0c91e5a5bc61c71da08d6968d973255829678d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23240/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23240?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23240      +/-   ##
==========================================
+ Coverage   92.22%   92.22%   +<.01%     
==========================================
  Files         169      169              
  Lines       50897    50899       +2     
==========================================
+ Hits        46940    46942       +2     
  Misses       3957     3957
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.65% <100%> (ø)` | :arrow_up: |
| #single | `42.29% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23240?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/23240/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `86.53% <100%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23240?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/23240?src=pr&el=footer). Last update [5a0c91e...699144b](https://codecov.io/gh/pandas-dev/pandas/pull/23240?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 looks ok. can you merge master, and does this need to go before #22862 ? >can you merge master, and does this need to go before #22862 ?

Just pushed.  22862 is PeriodArray, maybe you meant #23215?  Should be fine either way. thanks!