QUnit testrunner: Fix comment regarding animation and AJAX check

Follows-up b90c69cdf7.

This takes the approach of throwing an error to make the exact issue
obvious.

Unlike other versions (including the code in jQuery it is loosely
based on), this does not attempt to cleanup by stopping the effects
queue.

Change-Id: Ibb6ca0d083d4e8f1556e6f3493da17f7a5f44024
Merge "QUnit testrunner: Fix comment regarding animation and AJAX check"