Tests: expose 'args' passed to hook scripts

- Useful to validate that all the expected arguments were passed with
  correct escaping.
- Update test/template.t with one use-case for 'args'

test/template.t

@@ -157,6 +157,9 @@ sys.exit(0)
         # (according to python's JSON parser)
         hook.assertValidJSONOutput()
 
+        # Checking which arguments were passed to the hook
+        self.assertIn("/Hello/Greetings/", logs["calls"][0]["args"])
+
         # Some message output from the hook
         self.assertEqual(logs["output"]["msgs"][0],
                          "Hello from the template hook")