tests: enforce use of $PYTHON in tests

tests: use $PYTHON in #! so we always use the right Python

tests: sed away python #! in test-run-tests.t to avoid some upcoming insanity

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.

keyword: use context manager for rollback locking

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.

changegroup: delete "if True" and reflow

changegroup: let callers pass in transaction to apply() (API) I think passing in the transaction makes it a little clearer and more consistent with bundle2.