test_defaultBannerSentOnlyOnce fails intermittently in parallel tests

Almost identical:

======================================================================
ERROR: lp.services.sshserver.tests.test_auth.TestAuthenticationBannerDisplay.test_defaultBannerNotSentOnFailure
----------------------------------------------------------------------
Traceback (most recent call last):
_StringException: Empty attachments:
  twisted-log

Traceback (most recent call last):
  File "/var/lib/buildbot/dependencies/eggs/testtools-0.9.14-py2.6.egg/testtools/deferredruntest.py", line 199, in _log_user_exception
    raise e
UncleanReactorError: The reactor still thinks it needs to do things. Close all connections, kill all processes and make sure all delayed calls have either fired or been cancelled:
  <DelayedCall 0x12da756c [-36.6344602108s] called=0 cancelled=1>

Also:

======================================================================
ERROR: lp.services.sshserver.tests.test_auth.TestPublicKeyFromLaunchpadChecker.test_noKeys
----------------------------------------------------------------------
Traceback (most recent call last):
_StringException: Empty attachments:
  twisted-log

Traceback (most recent call last):
  File "/var/lib/buildbot/dependencies/eggs/testtools-0.9.14-py2.6.egg/testtools/deferredruntest.py", line 199, in _log_user_exception
    raise e
UncleanReactorError: The reactor still thinks it needs to do things. Close all connections, kill all processes and make sure all delayed calls have either fired or been cancelled:
  <DelayedCall 0xceafe8c [-106.44284296s] called=0 cancelled=1>

Bug 924378 looks related.

This error can also occur in lp.buildmaster.tests.test_manager.TestCancellationChecking.test_ignores_build_not_cancelling.

Fixed in stable r15053 (http://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable/revision/15053) by a commit, but not testable.