TestDNSStaticIPAddressListener test fails erroneously in CI

[Kit Randel]

maasserver.triggers.tests.test_system_listener.TestDNSStaticIPAddressListener.test_sends_message_for_update_on_dnsresource is potentially a bit flakey and fails erroneously in CI sometimes.

======================================================================
ERROR: maasserver.triggers.tests.test_system_listener.TestDNSStaticIPAddressListener.test_sends_message_for_update_on_dnsresource
----------------------------------------------------------------------
Traceback (most recent call last):
testtools.testresult.real._StringException: Twisted logs: {{{Listening for database notifications.Connection closed.}}}

Traceback (most recent call last):
  File "/run/build/maas/src/maastesting/runtest.py", line 138, in _run_user
    result = function(*args, **kwargs)
  File "/home/ubuntu/.buildout/eggs/testtools-2.2.0-py3.6.egg/testtools/testcase.py", line 719, in _run_test_method
    return self._get_test_method()()
  File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 461, in wrapper
    return eventual_result.wait(timeout)
  File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 231, in wait
    result.raiseException()
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 385, in raiseException
    raise self.value.with_traceback(self.tb)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
    result = g.send(result)
  File "/run/build/maas/src/maasserver/triggers/tests/test_system_listener.py", line 3321, in test_sends_message_for_update_on_dnsresource
    yield dv.get(timeout=2)
twisted.internet.defer.CancelledError

[Lee Trager (ltrager)]

I believe this is fixed as I can no longer reproduce it on master. Please reopen if you see it again.