TST: bare-bones fixture for timedelta array tests
Broken off from #23140.  The fixture needs to be fleshed out, but for now I just want to get it in place, since already other PRs need it.

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