mediawiki.loader: Clean up unit tests

mediawiki.loader: Clean up unit tests

* Avoid parenthesis in module name to make it easier to remember
  and type. Use "mediawiki.loader", which matches the naming for
  other mediawiki.js tests (e.g, 'mediawiki.html').

* Simplify teardown: Remove redundant check before deletion.
  The delete operator already performs this check and varies it
  returns value based on it (which we aren't using here).

* Remove 'Basic', which did the same as 'using() Promise'.

* Add test for 'using(, callback)' parameter.

* Simplify "did the script run" assertions by checking once
  afterwards, instead of checking both during the test and afterwards.
  The one during the test isn't a strict assertion, given it could
  be skipped. To preserve the detection of whether it ran twice,
  use a counter.

* Remove various redundant 'fail' assertions from Promise fail
  handlers in tests that were already returning said Promise,
  given QUnit already checks if the Promise is rejected.

* Simplify a few assert captions.

* Make assertion for 'messages load first' its own test,
  instead of happening in an unrelated test about @import CSS.

Change-Id: Icbb4ea7f16bb1f702fd92eb8007b7179d4763151