TST: Use new arithmetic fixtures, parametrize many more tests
Focus is mostly on test_datetime64.py to keep things close to orthogonal.

De-duplicates some tests that are in e.g. both TestFrameArithmetic and TestSeriesArithmetic
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/5625fd2cff9be1ac2861da1c814f284794f7c4ec/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/5625fd2cff9be1ac2861da1c814f284794f7c4ec/pandas/tests/arithmetic/test_datetime64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_numeric.py`](https://github.com/jbrockmendel/pandas/blob/5625fd2cff9be1ac2861da1c814f284794f7c4ec/pandas/tests/arithmetic/test_numeric.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_period.py`](https://github.com/jbrockmendel/pandas/blob/5625fd2cff9be1ac2861da1c814f284794f7c4ec/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/5625fd2cff9be1ac2861da1c814f284794f7c4ec/pandas/tests/arithmetic/test_timedelta64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_numeric.py`](https://github.com/jbrockmendel/pandas/blob/5625fd2cff9be1ac2861da1c814f284794f7c4ec/pandas/tests/indexes/test_numeric.py) !

 looks fine. lots of tests :-D > looks fine. lots of tests :-D

Yep, and you better believe we're not done yet.  Working on a test_timedelta64 branch for after #23642. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23757?src=pr&el=h1) Report
> Merging [#23757](https://codecov.io/gh/pandas-dev/pandas/pull/23757?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/24bce1a5fdd70a66b9fb5e2f9f51631d1df6add3?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23757/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23757?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23757      +/-   ##
==========================================
+ Coverage   92.23%   92.24%   +0.01%     
==========================================
  Files         161      161              
  Lines       51408    51408              
==========================================
+ Hits        47416    47422       +6     
+ Misses       3992     3986       -6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (+0.01%)` | :arrow_up: |
| #single | `42.3% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23757?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/23757/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.56% <0%> (+0.56%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23757?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/23757?src=pr&el=footer). Last update [24bce1a...d0ac7f2](https://codecov.io/gh/pandas-dev/pandas/pull/23757?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 