Bug 856006 - Try to fix intermittent failures in KeyEvents browser-element test. r=kk1ff

The intermittent failures are coming from events with keyCodes 34, 93,
and 95, corresponding to ", ], and _.  The test doesn't send any of
these keys; I don't know where they're coming from.  But at the very
least, we can not turn the test orange when we receive these events.

This change also removes some comments in the test about keypresses
being preventDefault'ed that, as far as I can tell, are incorrect.