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

Partially addresses #15990.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=h1) Report
> Merging [#16101](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/12f0762f65274544ef39304f7d6d7848087cd7e0?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/16101/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16101      +/-   ##
==========================================
+ Coverage   90.84%   90.84%   +<.01%     
==========================================
  Files         159      159              
  Lines       50778    50775       -3     
==========================================
- Hits        46128    46126       -2     
+ Misses       4650     4649       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <100%> (ø)` | :arrow_up: |
| #single | `40.34% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `79.23% <100%> (-0.06%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=footer). Last update [12f0762...147f153](https://codecov.io/gh/pandas-dev/pandas/pull/16101?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!