* Separate out tests for MediaWikiGadgetsDefinitionRepo and
Hooks::onGetPreferences. Removed hack in fetchStructuredList() for
overriding page content for tests.
* Add tests for ES6 gadgets. For this, ResourceLoaderModuleTest is
converted back to an integration test.
* Removed createGadgetDefinitionPage() test helper method. Now using
MediaWikiIntegrationTestCase::editPage().
* Fixed GDNamespaceRepoTest failure when GDNamespaceRepo is the active
repo, by clearing WAN process cache.
Change-Id: I26b84576a91f6cb0ebae64c5fc1408666d767911
