Fixed WAI-ARIA test related to #11821 which never has succeeded

Due to Testbench bug #12375 the test had never succeed on IE9/IE10.
To circumvent the issue I changed the way the test was done by
substituting the /@class assertion with an xpath statement instead which
works on all supported browsers.

Change-Id: Idd5b8cb37404a2f88c4a00dc9849e3645559551d