The WikibaseRepo.WikibaseRepo.BagOStuffSiteLinkConflictLookup service wiring (introduced in T291377) uses the ObjectCache::getLocalClusterInstance() function, which means that the unit test for that service wiring actually can’t run without MediaWiki being set up, because ObjectCache::getLocalClusterInstance() uses MediaWiki globals and the default service container.
A solution for this almost certainly requires T243233: MediaWiki should provide a LocalClusterObjectCache service.