Iec4ed4c8 made it so addCoreDBData() only gets called once. Which is
nice, except for the fact that it means any test that puts
'user_groups' into $this->tablesUsed is going to cause UTSysop to no
longer be a sysop, so any later test that expects it to be one is going
to mysteriously fail.
Fix is blocked by some tests (GatherTests, ApiEchoNotificationsTest, EchoDiscussionParserTest) relying on this (or at least on user tables not being cleaned up properly between tests).