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

Setup for #35964. There were two tests that are accidental duplicates; I've just copied them over to the transform modules. Will cleanup in #35964.