TST: groupby-reindex on DTI
Add test to check whether reindexing works correctly.

- [ ] closes #26209
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think need to run black on the change to get CI to pass > I think need to run black on the change to get CI to pass

done and now CI-linting passes Hello @CloseChoice! 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 2020-04-23 22:17:39 UTC thanks @CloseChoice 