TST: Split test_offsets.py - Test_offsets moved to test_date_offsets
In context of [https://github.com/pandas-dev/pandas/pull/30194](30194)
First part with move test_offsets to test_date_offsets
- [ ] closes #27085
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
