CLN: Split test_base
part of #23877

- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Some notes:
- `test_ops.py` still feels pretty bloated, but I didn't see a good opportunity for further splitting
- `test_conversion.py` is testing things like `to_list` and `to_numpy`. Maybe there's a better name for the file