BUG: fix tzaware dataframe transpose bug
This allows us to get rid of a bunch of xfails and FIXMEs in arithmetic tests.  Changes in groupby are the most likely to need attention; this is not an area of the code I know well.

Not sure if TestTranspose belongs somewhere else.  Suggestions welcome.

Split a couple of too-widely-scoped groupby tests.

Will follow-up with issues this closes, whatsnew entry, and GH references added to tests.

xref #23988

- [x] closes #13287
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
