TST: add regression test for MultiIndex with IntervalIndex level
#27456  makes for a good regression test for the non-overlapping case, where there's no coverage currently. Add it to the tests.

Hello @pilkibun! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-07-24 19:33:52 UTC thanks @pilkibun 