TST: Test reindex w/ multiindex df (GH21247)
- [x] closes #21247
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Whats new:
Add a test to `pandas/tests/indexes/multi/test_reindex.py`

