Intermittent failure: test_estimated_runtime_sets_start_time_when_unavailable

I had an intermittent test failure when trying to land a branch:

  http://maas-ci-jenkins.internal:8080/job/maas/job/branch-tester/1982/consoleText

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ERROR: metadataserver.models.tests.test_scriptresult.TestScriptResult.test_estimated_runtime_sets_start_time_when_unavailable
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Traceback (most recent call last):
testtools.testresult.real._StringException: Empty attachments:
  Twisted logs

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 "/run/build/maas/src/metadataserver/models/tests/test_scriptresult.py", line 484, in test_estimated_runtime_sets_start_time_when_unavailable
    self.assertEquals('0:00:00', script_result.estimated_runtime)
  File "/run/build/maas/src/maasserver/models/cleansave.py", line 118, in __getattribute__
    return super(CleanSave, self).__getattribute__(name)
  File "/run/build/maas/src/metadataserver/models/scriptresult.py", line 158, in estimated_runtime
    script_result.save(update_fields=['started'])
  File "/run/build/maas/src/metadataserver/models/scriptresult.py", line 376, in save
    return super().save(*args, **kwargs)
  File "/run/build/maas/src/maasserver/models/cleansave.py", line 203, in save
    return super(CleanSave, self).save(*args, **kwargs)
  File "/run/build/maas/src/maasserver/models/timestampedmodel.py", line 60, in save
    if self.pk is None or (_created is None and self.created is None):
  File "/run/build/maas/src/maasserver/models/cleansave.py", line 118, in __getattribute__
    return super(CleanSave, self).__getattribute__(name)
  File "/run/build/maas/src/maasserver/models/cleansave.py", line 69, in __get__
    "Accessing deferred field is not allowed: %s" % self.field_name)
maasserver.models.cleansave.DeferredValueAccessError: Accessing deferred field is not allowed: created