Fix the "SyncMutatedUpdateRecordTestCase" test
This test was written long before the feature was implemented and has been disabled since. The feature is now implemented but when the test is enabled, it fails with the following exception thrown:
pulp_rpm.app.exceptions.AdvisoryConflict: Incoming and existing advisories have the same id and timestamp but different and intersecting package lists. At least one of them is wrong. Advisory id: RHEA-2012:0055
This most likely means that Pulp 3 is more conservative than the test is designed for, and rejects the advisory in our test fixture because both versions of the advisory have identical timestamps but different package lists.
We need to find (or create) a fixture advisory where the two versions of the advisory have differing timestamps, so that the package lists can be successfully merged.