There are a lot of events that get dispatched onto the main queue
(often /from/ the main queue), and processing them during the setup
for the subsequent test can cause problems. In particular, the
notification for setting up a local number in -[OWSMessageManagerTest
test_GroupUpdate] was occasionally being processed in the subsequent
-test_GroupUpdateWithAvatar, depending on how long GRDB took to set up
in the second test.
