raise an AssertionError if do_barrier fails

This removes a lot of boilerplate code.
diff --git a/tests/bsn_ipmask.py b/tests/bsn_ipmask.py
index 65885f0..9f0838f 100644
--- a/tests/bsn_ipmask.py
+++ b/tests/bsn_ipmask.py
@@ -153,7 +153,7 @@
               self, pkt1, ing_port=ports[0], egr_ports=[ports[2]],
               wildcards=wildcards))
 
-        self.assertEqual(do_barrier(self.controller), 0, "Barrier failed")
+        do_barrier(self.controller)
             
         msg(ip0)
         self.dataplane.send(ports[0], str(pkt0))