TST: corrwith and tshift in groupby/groupby.transform
This PR tests `corrwith` and `tshift`, so not sure if it fully closes the original issue or not, but the other functions`backfill`, `pad` and `cumcount` don't seem to be supported in `groupby.transform` as reported in issues  #27472 and #31269.

- [x] closes #27905 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

