Collect datetime64 and PeriodDtype arithmetic tests
As suggested, these are split by dtype in a new tests/arithmetic directory.

Started parametrizing a few of these over `box` classes, but for the most part this is just collecting tests from their existing locations.  There is going to be a lot of parametrization and de-duplication coming up.

Holding off on moving the existing timedelta64-dtype tests until #22163 is handled to avoid merge conflict.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22237?src=pr&el=h1) Report
> Merging [#22237](https://codecov.io/gh/pandas-dev/pandas/pull/22237?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/737b3297e2d03492acbbe74eca5dfc2eb62cf207?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22237/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22237?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22237      +/-   ##
==========================================
+ Coverage   92.06%   92.07%   +<.01%     
==========================================
  Files         169      169              
  Lines       50698    50698              
==========================================
+ Hits        46676    46680       +4     
+ Misses       4022     4018       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22237?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22237/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.9% <0%> (+0.2%)` | :arrow_up: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/22237/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `91.98% <0%> (+0.76%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22237?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/22237?src=pr&el=footer). Last update [737b329...7ab3305](https://codecov.io/gh/pandas-dev/pandas/pull/22237?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks.

thought was moving ``pandas/tests/test_arithmetic.py`` as well?

maybe ought to move some of the fixtures to the top-level conftest > thought was moving pandas/tests/test_arithmetic.py as well?

That's coming up, but I decided to wait on that until after #22163 to avoid merge conflicts