TST: fix fixture for numpy dtypes
- [x] split off from #23982
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This was one of the things I stumbled over while working on #23982. There's a fixture for all_numpy_dtypes that got introduced in #21899, but it's a lie - it doesn't cover (nearly) all numpy dtypes. It's currently only used in one test, and so I've adapted it here to be able to actually test all numpy dtypes.
