Fix intermittent QUnit failure

Fix intermittent QUnit failure

Occasionally the test 'mediawiki: mw.loader.implement( styles={ "url": {
<media>: [url, ..] } } )' will fail because it only ran 5 of 7 expected
assertions, while at the same time a later test will fail because it had
two extra assertions.

The problem is that the test is performing two external CSS loads, but
calls QUnit.start() after the ''first'' one is loaded. If the second
doesn't manage to get loaded at close to the same time, QUnit moves on
to the next test, and then when the second external CSS file finally
does load it will get counted towards whichever test is running at that
time. This can be reliably reproduced by arranging for
tests/qunit/data/styleTest.css.php to take longer for one than the other
of these two external CSS loads.

The solution is to wait until *both* of those external CSS loads to
complete (or time out) before calling QUnit.start().

Change-Id: Id695e95733b4c3f58234d9688c6b6e1b9ba591cc