EntityRevisionLookupTest must use mock databases.

WikiPageEntityRevisionLookupTest extends EntityRevisionLookupTest,
but until now, EntityRevisionLookupTest did not extend MediaWikiTestCase.
Because of this, @group Database had no effect, and the tests were
executed the "real" database.

Change-Id: I616da6b3c2ad13252e92b4e428e2aa097a50dab8
Merge "EntityRevisionLookupTest must use mock databases."