raise an AssertionError if do_barrier fails

This removes a lot of boilerplate code.
diff --git a/tests/flow_expire.py b/tests/flow_expire.py
index ece6dcd..484f22f 100644
--- a/tests/flow_expire.py
+++ b/tests/flow_expire.py
@@ -68,7 +68,7 @@
         
         logging.info("Inserting flow")
         self.controller.message_send(request)
-        self.assertEqual(do_barrier(self.controller), 0, "Barrier failed")
+        do_barrier(self.controller)
 
         (response, pkt) = self.controller.poll(exp_msg=ofp.OFPT_FLOW_REMOVED,
                                                timeout=test_timeout)