TST: added test_join_multiindex_dates
- [ ] closes #33692
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

Added tests to make sure that  joining dataframes with multiindices containing dates doesn't break.
