mediawiki.user: Minor clean up of code and unit tests

mediawiki.user: Minor clean up of code and unit tests

* Clarify getRegistration() return documentation and minor
  optimisation of code order.

* Fix typo in test name for getUserInfo().

* Remove redundant test expect numbers.
  Follows-up 7c363752ed.

* Separate getter assertions for anonymous and logged-in.
  Avoid changing the fixture mid-test. Let setup/teardown
  clean up in the middle.

Change-Id: If777d44a571ac590e91ed84060ea714df67f8f2f