MAINT: Remove assertNotIn from testing
Title is self-explanatory.

Partially addresses #15990.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=h1) Report
> Merging [#16096](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/19fc8dac68e088126ffd132dc322dbf8a163ec69?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/16096/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16096      +/-   ##
==========================================
+ Coverage   90.83%   90.84%   +<.01%     
==========================================
  Files         159      159              
  Lines       50783    50778       -5     
==========================================
  Hits        46131    46131              
+ Misses       4652     4647       -5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <100%> (ø)` | :arrow_up: |
| #single | `40.35% <71.42%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.29% <100%> (-0.1%)` | :arrow_down: |
| [pandas/\_version.py](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=tree#diff-cGFuZGFzL192ZXJzaW9uLnB5) | `44.65% <0%> (+1.9%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16096?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/16096?src=pr&el=footer). Last update [19fc8da...70fcd59](https://codecov.io/gh/pandas-dev/pandas/pull/16096?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!