TST: Move groupby tests out of test_function
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [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
- [ ] 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.

The tests that remain are tests that go across reductions, transformations, and filters. The bulk of these are tests for `numeric_only`
tests arent changed, just cut/paste? > tests arent changed, just cut/paste?

Yea - that's right. Just a move here. Once `test_functions` is no more and `test_groupby` only has tests for the `GroupBy` instances themselves, I'm going to start trying to improve the quality of individual tests. Thanks @rhshadrach 