Fix management queue test errors

These tests were missing a synchronization point making them subject
to race conditions.

Change-Id: I6103298a142aa34c4d73f0463e199655c68897fd

tests/test_scheduler.py

@@ -3241,6 +3241,7 @@ class TestScheduler(testtools.TestCase):
             self.assertEqual(
                 enqueue_times[str(item.change)], item.enqueue_time)
 
+        self.waitUntilSettled()
         self.worker.release('.*-merge')
         self.waitUntilSettled()
         self.worker.release('.*-merge')
@@ -3307,6 +3308,7 @@ class TestScheduler(testtools.TestCase):
         r = client.promote(pipeline='gate',
                            change_ids=['3,1'])
 
+        self.waitUntilSettled()
         self.worker.release('.*-merge')
         self.waitUntilSettled()
         self.worker.release('.*-merge')