Deprecation warning breaks Node model tests

When I run the Node model tests in isolation, no matter how many, I get exactly one failure like:

«
FAIL: maasserver.models.tests.test_node.NodeManagerTest_StartNodes.test__sets_user_data
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "src/maasserver/models/tests/test_node.py", line 1622, in test__sets_user_data
    self.assertEqual("", twisted_log.dump())
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 321, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 406, in assertThat
    raise mismatch_error
MismatchError: !=:
reference = u''
actual = u'src/metadataserver/fields.py:58: exceptions.DeprecationWarning: object.__init__() takes no parameters'
»

A lot of model tests check the Twisted error log, and aren't expecting to find this DeprecationWarning there. It looks as if the warning only comes once per test run, and so whatever test does this check first will fail. The others are unaffected.