MAINT: Remove tm.TestCase from testing
Title is self-explanatory.

xref #15990
xref <a href="https://github.com/pandas-dev/pandas/pull/16201#pullrequestreview-36158775">#16201 (comment)</a>

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=h1) Report
> Merging [#16225](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4a748ccf6086a07e5ce18066099c835b2beef31f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16225/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16225      +/-   ##
==========================================
- Coverage   90.25%   90.23%   -0.03%     
==========================================
  Files         162      162              
  Lines       50882    50879       -3     
==========================================
- Hits        45925    45912      -13     
- Misses       4957     4967      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.02% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.31% <100%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `78.87% <ø> (-0.11%)` | :arrow_down: |
| [pandas/conftest.py](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree#diff-cGFuZGFzL2NvbmZ0ZXN0LnB5) | `95.83% <100%> (+0.37%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.58% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16225?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/16225?src=pr&el=footer). Last update [4a748cc...70297b6](https://codecov.io/gh/pandas-dev/pandas/pull/16225?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!