On chutney verify failure, inform user about debug mode

When chutney verify fails, tell the user how to activate debug mode
by editing the source code.

Eventually, we'll make debug mode an argument, see #15418.

lib/chutney/TorNet.py

@@ -899,6 +899,10 @@ class Network(object):
         print("Verifying data transmission:")
         status = self._verify_traffic()
         print("Transmission: %s" % ("Success" if status else "Failure"))
+        if not status:
+            # TODO: allow the debug flag to be passed as an argument to
+            # src/test/test-network.sh and chutney
+            print("Set 'debug_flag = True' in Traffic.py to diagnose.")
         return status
 
     def _verify_traffic(self):