tests: Add test for testing that setting a parent affects the stack

A new test is added that tests that xdg_surface.set_parent (referred to
as "transient for" in X11 terminology) affects the stack immediately.

https://bugzilla.gnome.org/show_bug.cgi?id=755606

src/tests/test-runner.c

@@ -767,6 +767,23 @@ test_case_do (TestCase *test,
                            NULL))
         return FALSE;
     }
+  else if (strcmp (argv[0], "set_parent") == 0)
+    {
+      if (argc != 3)
+        BAD_COMMAND("usage: %s <client-id>/<window-id> <parent-window-id>",
+                    argv[0]);
+
+      TestClient *client;
+      const char *window_id;
+      if (!test_case_parse_window_id (test, argv[1], &client, &window_id, error))
+        return FALSE;
+
+      if (!test_client_do (client, error,
+                           "set_parent", window_id,
+                           argv[2],
+                           NULL))
+        return FALSE;
+    }
   else if (strcmp (argv[0], "show") == 0 ||
            strcmp (argv[0], "hide") == 0 ||
            strcmp (argv[0], "activate") == 0 ||