If you were writing a government test case then you would use Shall. Otherwise, it really shouldn't matter. The exception is that Must indicates an action meaning that you must disable it yourself. Which is really not the intent. You mean to verify that it is disabled. So why not say that, for clarity's sake.
1. Cancel Button indicates a disabled condition.
Success is the ability to go from one failure to another with no loss of enthusiasm.
~ Winston Churchill ~