[2.2] Random test failure: test_dehydrate_script_set_status

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FAIL: maasserver.websockets.handlers.tests.test_machine.TestMachineHandler.test_dehydrate_script_set_status
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Traceback (most recent call last):
testtools.testresult.real._StringException: Empty attachments:
  Twisted logs

Traceback (most recent call last):
  File "/tmp/tarmac/branch.2zPEjH/src/maastesting/runtest.py", line 134, in _run_user
    result = function(*args, **kwargs)
  File "/home/ubuntu/.buildout/eggs/testtools-2.2.0-py3.5.egg/testtools/testcase.py", line 719, in _run_test_method
    return self._get_test_method()()
  File "/tmp/tarmac/branch.2zPEjH/src/maasserver/websockets/handlers/tests/test_machine.py", line 966, in test_dehydrate_script_set_status
    handler.dehydrate_script_set_status(script_result.script_set))
  File "/home/ubuntu/.buildout/eggs/testtools-2.2.0-py3.5.egg/testtools/testcase.py", line 411, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/home/ubuntu/.buildout/eggs/testtools-2.2.0-py3.5.egg/testtools/testcase.py", line 498, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: 4 != 3