TEST: join df with categorical multiIndex
- [x] closes #50906
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Add test to check index dtype after joining DataFrames with categorical multiIndex.
Thank you, I am looking forward to seeing a cat :) this is currently marked as draft:

![image](https://user-images.githubusercontent.com/33491632/216652138-8cc3125b-aba2-478d-bab2-e72e70bf678d.png)

it looks ready though, anything else need doing? if you mark as ready for review I think we can ship it I forgot to change the status of the PR. Now it is "ready for review" :)