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