tests: remove WarnTest
We would raise this if a test didn't return a result code. AFAICT
this can only occur if there is a logic error in the test harness
itself.
I don't think it is worth the code complexity to distinguish this
failure scenario from a regular test failure.
tests: remove unused IgnoreTest exception
AFAICT its last use was removed in
d839e4820da7.
tests: use unittest.SkipTest
unittest.SkipTest was introduced in Python 2.7. We previously defined
it with our own class so we could run on Python 2.6.