Bug 829820 - Ability to mark selected SMS thread as Read/Unread

Since the list selection is no longer depends on DOM status, you have to change ThreadListUI.selectionHandler.selected manually for unit test. For this case, you will need to reset ```ThreadListUI.selectionHandler = null``` at setup and ThreadListUI.selectionHandler.selected = new Set(['1', '2']); for selecting thread 1 and 2.