Add a test for agg std
- [X] closes #18734 
- [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).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.


I believe that this test ensures the expected output. Let me know if you have any questions or concerns.
Can you move the test to `pandas/tests/apply/test_frame_apply.py`? I believe that this will fulfill your request, but please let me know if there is anything else I can do. @mroeschke  Looks like a few of the code check builds are failing @mroeschke , I have fixed the code checks that were failing, and I have constructed the tests  as  your instructions.  Awesome! Thanks @steliospetrakis02  > Awesome! Thanks @steliospetrakis02

You're welcome! I thank you for your guidance and assistance as I take my first steps into contributing to pandas