Follows-up I24af0c2fa75. I forgot that newEmpty uses the "always empty"
EmptyBag, as opposed to an "initially empty" HashBag.
Change-Id: I62075bf685223b31c161508f99f95ee46280450b
* 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