maasserver.models.tests.test_bootsource.TestBootSource.test_calls_cache_boot_sources_on_create sometimes fails spuriously

[Graham Binns]

Running make check, I saw this:

======================================================================
FAIL: maasserver.models.tests.test_bootsource.TestBootSource.test_calls_cache_boot_sources_on_create
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_StringException: Traceback (most recent call last):
  File "/home/graham/workspace/maas-work/maas/src/maasserver/models/tests/test_bootsource.py", line 108, in test_calls_cache_boot_sources_on_create
    1, deferToThread, cache_boot_sources))
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 406, in assertThat
    raise mismatch_error
MismatchError: Expected to be called once. Called 2 times.

Re-running make check, the error didn't reappear. Nor does it appear when running the test in isolation