github: gracefully handle unknown event

Return an empty 200 for unknown event.

Change-Id: Ic1ac932b8d9926f1a9e9071774a59e8429f702ae
diff --git a/tests/base.py b/tests/base.py
index 6393446..f2c0275 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -922,7 +922,7 @@
             'http://localhost:%s/connection/%s/payload'
             % (port, self.connection_name),
             data=payload, headers=headers)
-        urllib.request.urlopen(req)
+        return urllib.request.urlopen(req)
 
     def getPull(self, project, number):
         pr = self.pull_requests[number - 1]