Bug 1086277 - Add a test for add & delete a divider on homescreen

What if we fall in the following case?
1. The first app is rendered, but we're still waiting for the others
1. The initial_number_of_dividers is set to 0 then.
1. The others apps and dividers are rendered and the actual number of dividers is now `N`
1. The assertion `self.assertEqual(initial_number_of_dividers + 1, len(self.homescreen.divider_elements))` won't pass, will it?