TST: fix test for MultiIndexPyIntEngine on 32 bit
- [x] closes #19439
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

It was just a problem in the test.
The failure should be unrelated to this PR thanks. will merge after PR to fix travis is merged. thanks! https://travis-ci.org/MacPython/pandas-wheels/jobs/334972407

is get_indexer returning platform ints? (it should)

no need to create a new issue, can just issue a PR I fixed only one of the two checks... sorry, PR coming.