Fix intermittent failures in test suite
We often see this intermittent failure in the test suite, usually with the MySQL build:
test_master_is_elsewhere_and_findable (mailman.commands.tests.test_control.TestBinDir) ... WARNING: SIGTERM DID NOT EXIT PROCESS; SIGKILLing
ok
test_force_stale_lock (mailman.commands.tests.test_control.TestStart) ... FAIL
Most of the time a retry will solve the problem, but this really isn't good enough. Let's try to figure out exactly what's going on and fix it so the test is reliable.