TST: Test groupby method drop na
- [x] closes #21755 (Replace xxxx with GitHub issue number) 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) when a bug was fixed or a new feature was added - [ ] All [Code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) 
- [ ] [Type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) added to new arguments/methods/functions 
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file when a bug was fixed or a new feature was added.

referred to [TST: Added test for groupby dropping NA when grouping by columns from pbhoopala](https://github.com/pandas-dev/pandas/pull/50156)
Thanks @luke396 