Bug 1053964 - [Messages][Tests] Add marionette JS test case to verify close\back button behaviour when app is in activity mode. r=julien

Reviewing gaia-header patch yesterday I've learned that it's actually possible to break Thread panel "close" button behavior when leaving Report panel, and fix it by going to Participants panel afterwards :) So, now I'm worried that we can miss such regressions if we join test cases.