Some tests for WikibaseQualityConstraints fail on quibble-vendor-mysql-hhvm-docker with the error message:
PHPUnit_Framework_MockObject_RuntimeException: The provided class "Mock_WANObjectCache_157858e2" does not exist
This is blocking merges.
Example failure: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/525234