MediaWikiTestCase: Fix schema restoration

MediaWikiTestCase: Fix schema restoration

First, after restoring the old schema it needs to clear the variable so
it doesn't keep "restoring" again and again for every subsequent test
class.

Second, after restoring the old schema it needs to repopulate anything
added by addCoreDBData() that was lost by dropping and recreating the
affected tables, which is easiest done by calling $this->resetDB().

Change-Id: I549d10eb7ceec117d3a0138449ca01f8065707d4