TST: GH28813 test .diff() on Sparse dtype
- [x] closes #28813 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Add a quick test to confirm that .diff() function handles sparse dataframes the same way as regular dataframes. 

First time making a pandas PR so comments, advice, critiques are encouraged.