Update test_music to fix early failure

Change xfail reason to the correct reason

This test still needs to be xfailed, but the reason previously stated was inaccurate. I also added a wait that resolved an intermittent failure that was ocurring before the code for which the test is being xfailed.

The problem is https://bugzilla.mozilla.org/show_bug.cgi?id=862156, which is both preventing us from waiting for the proper element to be displayed, and also preventing us from tapping on the button even if we wait via sleep(), which we were doing before.