qunit: Abort unfinished AJAX requests in test teardown

qunit: Abort unfinished AJAX requests in test teardown

Similar to what we do with animations already, ensure we give
the next test a clean start by aborting any requests that were
made during the test that are still pending.

Also log the details of the request (ajax options, e.g. url) to
the console.

To test:
* Add "sleep( 1 );" to LocalSettings.php.
* add "$.ajax( mw.util.wikiScript() )" to a test in mediawiki.util.test.js.
* Run Special:JavaScriptTest/qunit/plain?module=mediawiki.util

Change-Id: Iefef89effc092d296baa9df68a86d95497730708