provisioningserver TestGetSourceAddress.test_returns_none_if_no_route_found sometimes fails locally

[Igor Brovtsin]

I tried running `make test` locally and the "provisioningserver.utils.tests.test_network.TestGetSourceAddress.test_returns_none_if_no_route_found" test failed with the following traceback on master/0d37f91f4dd233548fb78cde64823168e95caedb:

Traceback (most recent call last):
  File "/work/src/provisioningserver/utils/tests/test_network.py", line 2474, in test_returns_none_if_no_route_found
    self.assertThat(get_source_address("127.0.0.0"), Is(None))
  File "/work/.ve/lib/python3.10/site-packages/testtools/testcase.py", line 480, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: '127.0.0.1' is not None

The environment is Ubuntu 22.04 LXD container.