test-runner: Always wait after creating a window

Creating a window could take some time, causing false-positive failures when
running in slower or busy hardware like:

  window 1/2 isn't known to Mutter

So before we proceed in doing any operation on it, wait for the client.
Do this in the test runner instead of repeating the same in every .metatest.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/581

src/tests/test-runner.c

@@ -385,6 +385,9 @@ test_case_do (TestCase *test,
                            argc == 3 ? argv[2] : NULL,
                            NULL))
         return FALSE;
+
+      if (!test_client_wait (client, error))
+        return FALSE;
     }
   else if (strcmp (argv[0], "set_parent") == 0 ||
            strcmp (argv[0], "set_parent_exported") == 0)