Fix race in gerrit+github test

These builds could otherwise arrive out of order.

Change-Id: Ifc61cb17a62a8c6488d05dec37e69b7908ad7673
diff --git a/tests/unit/test_multi_driver.py b/tests/unit/test_multi_driver.py
index a1107de..864bd31 100644
--- a/tests/unit/test_multi_driver.py
+++ b/tests/unit/test_multi_driver.py
@@ -28,10 +28,10 @@
 
         A = self.fake_gerrit.addFakeChange('org/project', 'master', 'A')
         self.fake_gerrit.addEvent(A.getPatchsetCreatedEvent(1))
+        self.waitUntilSettled()
 
         B = self.fake_github.openFakePullRequest('org/project1', 'master', 'B')
         self.fake_github.emitEvent(B.getPullRequestOpenedEvent())
-
         self.waitUntilSettled()
 
         self.assertEqual(2, len(self.builds))