TST: 26807 split pandas/tests/tseries/offsets/test_offsets.py into multiple smaller test modules
This is to address xref #26807 specifically for pandas/tests/tseries/offsets/test_offsets.py . I tried to get all the new modules below about 1000 lines and break it up in logical ways.

Other than moving code, I have changed some list construction for pytest parameterizing to be a tiny bit more compact by changing a bunch of sequential `append`s to a single list literal. Other than that no changes.

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