TST: add tests for period resample sum with min_count
- [x] closes #19974
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

can you merge master and will look again @jreback - rebased onto latest master. @ganevgv - sorry can you merge master again - there are some conflicts superseded by #31196