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

It passed one time today for me without any changes, but then failed again... I discover that it always stops at recipientList.sendKey action. So I add waitfor placefolder element before recipientList.sendKey, and everything is fine then. Do you think it would be safer to do so? It' s quite frustration if our patch blocked by intermittent integration testing error in the future.