resourceloader: Improve ResourceLoaderWikiModule test coverage

resourceloader: Improve ResourceLoaderWikiModule test coverage

* Remove redundant getContent() cases that were all testing the same.
  The redirect logic should indeed be tested, but exists in getContentObj(),
  not getContent(). This test was also mocking getContentObj() thus not
  actually testing what the case claims to test.
  Fortunately, the right test already exists (testGetContentForRedirects),
  so this is redundant.

* Add actual coverage of successful outcomes for getContent (previously
  they were all error/null cases), with JS content, and with CSS content.

* Fix broken test case for "Bad content model". This was not working because
  it mocked out getContentObj, thus it wasn't actually testing "bad content model",
  but rather pointlessly duplicated the previous test case.
  Fix it by actually making it use a WikitextContent object, which makes it
  test the branch that handles incompatible content models.

Change-Id: I59af5318e536c730755352e9be8f995df1f56a86