Run isort on tests/indexes
- [ ] xref #23334
- [ ] tests added / passed
          * `pytest pandas/tests/indexes/interval/test_*`
          * `pytest pandas/tests/indexes/timedeltas/test_*`
          * `pytest pandas/tests/indexes/multi/test_*`
          * `pytest pandas/tests/indexes/test_*`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

