TST: added test for df.loc modify datetime columns
relevant GitHub issue #28837

- [x] closes #28837 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@mroeschke @simonjayhawkins 
Please review Hello @rohitsanj! 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 2019-10-29 04:56:10 UTC @mroeschke Hey, I realised I hadn't written the test case properly. I've updated it now. Can you check it out? Thanks for the commit @gfyoung Thanks @rohitsanj, @gfyoung!  