TST: Unskip some Categorical Tests
closes https://github.com/pandas-dev/pandas/issues/20747
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_categorical.py`](https://github.com/TomAugspurger/pandas/blob/5d9a1cfb035379e8b8bd3d46e06a48d16150d823/pandas/tests/extension/test_categorical.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23613?src=pr&el=h1) Report
> Merging [#23613](https://codecov.io/gh/pandas-dev/pandas/pull/23613?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8ed92efc65b51a9010bd14e0e666fc722226e50a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23613/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23613?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23613   +/-   ##
=======================================
  Coverage   92.25%   92.25%           
=======================================
  Files         161      161           
  Lines       51277    51277           
=======================================
  Hits        47305    47305           
  Misses       3972     3972
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23613?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/23613?src=pr&el=footer). Last update [8ed92ef...afe6c83](https://codecov.io/gh/pandas-dev/pandas/pull/23613?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!