merge with stable

tests: don't touch the network when using virtualenv

tests: removed ReportedTest exception The only call site called addFailure before raising, which is exactly what the failure exception handler does. So this complexity is not needed. We have test coverage of this "server failed to start" scenario and nothing appeared to change.

tests: remove support for warned tests The previous changeset removed the last caller of addWarn(). So, we rip out that method and all the code related to tracking warned tests in the results system. There was even a comment saying we may want to fold warned tests into the "failed" state, which is what the previous changeset did.

contrib: add check-code rule to enforce non-use of `python` in tests

cleanup: use $PYTHON to run python in many more tests Spotted one of these, then wrote a check-code rule that caught them all. It will be the next change.

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.