tests: remove #! from primes.py in test-highlight.t
It's about to be a source of trouble, but removing it changes a ton of
test lines, so doing this change as a standalone commit.
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.