make testUpdateTimestamp return skip rather than just pass/passing.

Author: rouilj

test/test_memorydb.py

@@ -65,7 +65,7 @@ def setUp(self):
 
     # doesn't work for memory as it uses a mock for session db.
     def testUpdateTimestamp(self):
-        pass
+        self.skipTest("This test does is not implemented for memorydb.")        
 
 # vim: set filetype=python ts=4 sw=4 et si