test_event_processing.TestEventProcessing.test_insert_events is flaky

It looks like test_event_processing.TestEventProcessing.test_insert_events can sporadically fail when waiting for insert events to be processed. To make it more robust, we should wait for longer than 10 seconds. We should also add a small delay when looping inside wait_for_insert_event_processing() to keep system load low and reduce the risk of starving the other processes of resources.