mediawiki.api.test: Refactor to embrace async

mediawiki.api.test: Refactor to embrace async

This was using lots of synchronous hacks but those don't work in newer
versions of Sinon.JS (and only worked by accident up until now).

The test will still run pretty quick, but it will yield to some extend because
mw.Api and jQuery.ajax both create XHR objects with async=true.

* Remove redundant QUnit.expect() calls (deprecated).
* Make use of QUnit's new Promise-return feature.
  This simplifies some tests by no longer having to call async() or start()
  manually when only dealing with a single promise.

This blocks upgrade of Sinon.JS.

Change-Id: I08b08ea3c3e8b41f4aea50d86b2c9dc0f579ba67