TL;DR we might want to change the calendar app to make this less weird.

When we go to a view with an id urlparam, we load the event associated with that id here https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/js/views/event_base.js#L303. This eventually adds a `.loading` class to the event view here https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/js/views/event_base.js#L120 which gets removed here https://github.com/mozilla-b2g/gaia/blob/master/apps/calendar/js/views/event_base.js#L144. This is also where the ui gets injected with the data we just loaded.

I think that we should give `_updateUI` a callback and only remove the `.loading` class once the callback is invoked. Then, in our test, we can wait for the view to lose `.loading` before returning from the open `readEvent` view.