REF/TST: Add pytest idiom to test_testing
Splits test file into multiple new files and delegates remaining tests to the miscellaneous `test_util.py`
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/util/conftest.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_almost_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_almost_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_categorical_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_categorical_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_extension_array_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_extension_array_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_frame_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_frame_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_index_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_index_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_interval_array_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_interval_array_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_numpy_array_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_numpy_array_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_assert_series_equal.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_assert_series_equal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_util.py`](https://github.com/gfyoung/pandas/blob/d8720b690116d7b756cc3a0713453358793202c4/pandas/tests/util/test_util.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=h1) Report
> Merging [#24369](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6cf7d9e2606e82fd3934e1dac8119c43f5ee643?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/24369/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24369      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51832    51832              
==========================================
+ Hits        47839    47840       +1     
+ Misses       3993     3992       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24369/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.75% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24369?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/24369?src=pr&el=footer). Last update [f6cf7d9...d8720b6](https://codecov.io/gh/pandas-dev/pandas/pull/24369?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/24369?src=pr&el=h1) Report
> Merging [#24369](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5d134ec1a9ab118abbd16157bd161783547117e6?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/24369/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24369      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51841    51841              
==========================================
+ Hits        47847    47849       +2     
+ Misses       3994     3992       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.98% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24369/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.75% <0%> (+0.18%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24369?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/24369?src=pr&el=footer). Last update [5d134ec...aca6404](https://codecov.io/gh/pandas-dev/pandas/pull/24369?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung 