Use @patch decorator instead of ExitStack context manager. This is more
consistent with the rest of the testsuite we have in britney.
Use some more idiomatic assertions, e.g.
unittest.mock.assert_not_called().
It works like this. We wait until all tests have finished running. and
then grab their results. If there are any regressions, we mail each bug
with a link to pending-sru.html. There's a state file which records the
mails we've sent out, so that we don't mail the same bug multiple times.
