REGR: groupby fails with nullable dtypes and dropna=False
- [x] closes #48794 (Replace xxxx with the Github issue number)
- [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).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This reworked test is a bit ghastly, any suggestions on how to simplify or clean up are most welcome.
Thanks @mroeschke - I was able to simplify this a bit (namely - remove an xfail, less parameters for determining sequence, less repetition of uniques). Also added back in `test_series` and sparse (thanks for catching that). What do you think - would you still like to see it broken up? Looks better. There's still a bit to grok to understand `range(3**4)` -> `xyz` -> unique values per dtype, but I don't think making it simpler should block the fix. Thanks @rhshadrach 