mediawiki.user.test: Use FakeXHR and clean up

The test was quite vague (type array, contain '*') which was
because it made an actual API request and that means the actual
user and wiki configuration influence it, naturally.

Replaced with a mock (also speeds up the test), which lets us
do a more appropiate and strict unit test.

There was also a test for asserting mw.config 'wgUserGroups' and
ApiQueryUserInfo/prop=groups return the same thing. This had to
be removed. Not sure where this would belong (if anywhere). Both
OutputPage and ApiQueryUserInfo get it from User::getEffectiveGroups().

Change-Id: I052ad0b93e8cef7a27f7020411ba4665132675f5