TestPostgresListenerService test fails erroneously in CI

[Kit Randel]

maasserver.tests.test_listener.TestPostgresListenerService.test__calls_handler_on_notification_with_delayed_registration is potentially a bit flakey and fails erroneously in CI sometimes.

[Alberto Donato (ack)]

[2020-06-25T12:44:42.094Z] ERROR: maasserver.tests.test_listener.TestPostgresListenerService.test_calls_handler_on_notification
[2020-06-25T12:44:42.094Z] ----------------------------------------------------------------------
[2020-06-25T12:44:42.094Z] Traceback (most recent call last):
[2020-06-25T12:44:42.094Z] testtools.testresult.real._StringException: Twisted logs: {{{Listening for database notifications.Connection closed.}}}
[2020-06-25T12:44:42.094Z]
[2020-06-25T12:44:42.094Z] Traceback (most recent call last):
[2020-06-25T12:44:42.094Z] File "/run/build/maas/src/maastesting/runtest.py", line 126, in _run_user
[2020-06-25T12:44:42.094Z] result = function(*args, **kwargs)
[2020-06-25T12:44:42.094Z] File "/run/build/maas/.ve/lib/python3.6/site-packages/testtools/testcase.py", line 719, in _run_test_method
[2020-06-25T12:44:42.094Z] return self._get_test_method()()
[2020-06-25T12:44:42.094Z] File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 461, in wrapper
[2020-06-25T12:44:42.094Z] return eventual_result.wait(timeout)
[2020-06-25T12:44:42.094Z] File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 231, in wait
[2020-06-25T12:44:42.094Z] result.raiseException()
[2020-06-25T12:44:42.094Z] File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 385, in raiseException
[2020-06-25T12:44:42.094Z] raise self.value.with_traceback(self.tb)
[2020-06-25T12:44:42.094Z] File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
[2020-06-25T12:44:42.094Z] result = g.send(result)
[2020-06-25T12:44:42.094Z] File "/run/build/maas/src/maasserver/tests/test_listener.py", line 200, in test_calls_handler_on_notification
[2020-06-25T12:44:42.094Z] yield dv.get(timeout=2)
[2020-06-25T12:44:42.094Z] twisted.internet.defer.CancelledError