Bug 843780 - Fix SMS mockup for desktop testing r=borjasalguero

This script was created to facilitate development on desktops using with
FireFox. It has broken since being implemented. This commit restores the
intended functionality and prevents race conditions in message creation.
Merge pull request #8246 from jugglinmike/fix-sms-mock

Bug 843780 - Fix SMS mockup for desktop testing