COMPAT: xarray 0.8.2 test compat w.r.t. CategoricalIndex idempotency
closes #15282

@shoyer  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15285?src=pr&el=h1) Report
> Merging [#15285](https://codecov.io/gh/pandas-dev/pandas/pull/15285?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c26e5bbed826aaf9a643c604f7f754f29af4bd64?src=pr&el=desc) will **increase** coverage by `-0.01%`.


```diff
@@            Coverage Diff             @@
##           master   #15285      +/-   ##
==========================================
- Coverage   86.33%   86.32%   -0.01%     
==========================================
  Files         139      139              
  Lines       51149    51149              
==========================================
- Hits        44157    44156       -1     
- Misses       6992     6993       +1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15285?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/c26e5bbed826aaf9a643c604f7f754f29af4bd64...0ad7b94e2159e5ad9083c69f3a36d078b56b4c2e?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <ø> (-0.34%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15285?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/15285?src=pr&el=footer). Last update [c26e5bb...0ad7b94](https://codecov.io/gh/pandas-dev/pandas/compare/c26e5bbed826aaf9a643c604f7f754f29af4bd64...0ad7b94e2159e5ad9083c69f3a36d078b56b4c2e?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).