TST: Dtype constants into pandas._testing to avoid direct imports from conftest
part of #30914

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

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Took a naive approach to move these constants into `pandas._testing`. Could also move them to a separate module instead. Let me know what you think is best.