[Panda] Wait for label ready in test_clock_set_alarm_label

The javascript in this Clock app sets the text of this label to be "Alarm" during the init of the element.

I believe the panda/marionette was setting the text before the js and thus it overwrote the work of marionette. Now it will wait for the field to be ready before sending keys.

Calling out @malini and @jonallengriffin to test this on Panda.

I have tested it on Panda and Unagi already.