added tests for the test_groupby_dropna.py
- [ ] closes #46783
Hello @hhheidi! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/tests/groupby/test_groupby_dropna.py`](https://github.com/pandas-dev/pandas/blob/b467389dc1eb095b5c0d094fd43e73466cc1259d/pandas/tests/groupby/test_groupby_dropna.py):

> [Line 352:1](https://github.com/pandas-dev/pandas/blob/b467389dc1eb095b5c0d094fd43e73466cc1259d/pandas/tests/groupby/test_groupby_dropna.py#L352): [E302](https://duckduckgo.com/?q=pep8%20E302) expected 2 blank lines, found 1
> [Line 363:38](https://github.com/pandas-dev/pandas/blob/b467389dc1eb095b5c0d094fd43e73466cc1259d/pandas/tests/groupby/test_groupby_dropna.py#L363): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 380:45](https://github.com/pandas-dev/pandas/blob/b467389dc1eb095b5c0d094fd43e73466cc1259d/pandas/tests/groupby/test_groupby_dropna.py#L380): [W292](https://duckduckgo.com/?q=pep8%20W292) no newline at end of file

 This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to be stale and is failing a lot of the CI checks. If interested in continuing, please fix the formatting and tests failures locally and we can reopen.