Bug 1104007 - add mochitests task for b2g emulator

Taskcluster results are coming in as of submitting this: 
http://docs.taskcluster.net/tools/task-graph-inspector/#A4yH06dGS-qJNDQwVLlBsA/6Brgk4lbTiKquZWc7gO7Dg/0