mw.loader: Avoid use of deprecated QUnit.asyncTest/QUnit.start

mw.loader: Avoid use of deprecated QUnit.asyncTest/QUnit.start

Deprecated since QUnit 1.16, removed in QUnit 2.0. (We're on 1.23 currently.)

Migrate to assert.async().

This is a fairly atypical use of QUnit.start(), because it functions here
as a cross-script callback, where lexical scope cannot be used to share
the async() callback directly.

Other mw.loader tests already solved this by using a static callback instead
which inherits the lexical scope from the test to call done(). The old
'qunitOkCall' script is no longer used after this and thus removed.

Change-Id: I430df14b35a69c71df8685494d1379e22af0d6df