TST: Check merging on equivalent CategoricalDtype
Closes #22501
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=h1) Report
> Merging [#25898](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/882961df6025b800b899d351c243412e799accf4?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/25898/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25898      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         175      175              
  Lines       52808    52808              
==========================================
+ Hits        48338    48339       +1     
+ Misses       4470     4469       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <ø> (ø)` | :arrow_up: |
| #single | `41.82% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25898/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.56% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25898?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/25898?src=pr&el=footer). Last update [882961d...b693f2d](https://codecov.io/gh/pandas-dev/pandas/pull/25898?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/25898?src=pr&el=h1) Report
> Merging [#25898](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/882961df6025b800b899d351c243412e799accf4?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/25898/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25898      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         175      175              
  Lines       52808    52808              
==========================================
+ Hits        48338    48339       +1     
+ Misses       4470     4469       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <ø> (ø)` | :arrow_up: |
| #single | `41.82% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25898/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.56% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25898?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/25898?src=pr&el=footer). Last update [882961d...69eda22](https://codecov.io/gh/pandas-dev/pandas/pull/25898?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!