Fix global objects and singletons in NewParserTest

- Don't set $wgMemc, $parserMemc and $messageMemc on each test,
  the correct values for $wgMainCacheType and such are already set
  in phpunit.php, so those objects are already the good ones
- Only destroy RepoGroup and FileBackendGroup once before starting
  the tests instead of two
- No need to clear the MessageCache in addDBData(), it is already
  destroyed in setupGlobals() (also avoids creating an object on
  some tests to only call clear() on it)
- Group singleton cleanup after the test in tearDown()

Change-Id: Ib2d6522019160f63f3f968e69940413a1def2f8b
Merge "Fix global objects and singletons in NewParserTest"