This switches most tests over to use pytest as the runner instead of unittest (taking
advantage of the fact that pytest can run unittest based tests).

There were a couple tests that had failures when swithing to pytest:
config/tests/unit-expandlibs.py
xpcom/idl-parser/xpidl/runtests.py

For these tests, I added a runwith='unittest' argument so that they still run the
same way as before. Once we fix them to use pytest, the unittest logic in mozunit.py
can be deleted.

MozReview-Commit-ID: Gcsz6z8MeOi

--HG--
extra : rebase_source : 3c762422ce0af54cbbe7d9fc20085a2d1ebe7057