Clean up the functorch test skip mechanism; add a new decorator (#85564)
This PR:
- adds a `decorate` thing that can be added to skip/xfail lists. This
lets people provide their own decorator (e.g. unittest.skipIf blah)
- does some refactoring of the skip/xfail list mechanism to make it
more sane
Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85564
Approved by: https://github.com/samdow
