Bug 1034279 - USSD messages not displayed if user leaves Dialer while sending r=etienne

I took the opportunity to re-enable a unit test

(cherry picked from commit 46bad9d7f31cbc79a4551b011529156ba8f1f8c7)

Conflicts:
	apps/communications/dialer/test/unit/mmi_ui_test.js

Signed-off-by: Ryan VanderMeulen <rvandermeulen@mozilla.com>