[WIP] TST/MAINT: split up test_resample.py (GH17806)
- [ ] closes #17806 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@winklerand love for you to continue on this. pls rebase when you can. can you rebase this closing as stale. if you want to continue working, pls ping.