TST: Split test_offsets.py
- [x] closes #27085
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@Raalsky can you rebase closing as stale. @Raalsky would love a PR that first moves, then followups to split / parameterize this. make small targeted PRs; we can then merge quickly. Sorry for being off during the holidays. I've prepared the first step (move) test_offsets to test_date_offsets #31031 Will be following by adding fixtures and then business/date split. Fixtures added in #31034 