Add pytest-flakefinder as a test dependency (#89103)
This is used to re-run tests multiple times to determine their flakiness status. The way re-run is handled in https://github.com/pytorch/pytorch/pull/88646 only applies to unittest
Per their documentation, `pytest-repeat` doesn't work with `unittest.Testcase` it seems, so trying https://github.com/dropbox/pytest-flakefinder instead
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89103
Approved by: https://github.com/clee2000
