TST: Add test of DataFrame.xs() with duplicates (#13719)
Added tests of DataFrame.xs() method for multilevel index containing duplicates.  The underlying bug reported in #13719 was fixed previously, but was not covered by specific unit tests.

- [ ] closes #13719
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
