Adding test_map_missing_mixed to test_apply.py in pandas test suite series
Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #20495
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (n/a)
