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/stacking/set-parent.metatest

@@ -0,0 +1,14 @@
+new_client 1 wayland
+create 1/1
+show 1/1
+create 1/2
+show 1/2
+wait
+assert_stacking 1/1 1/2
+
+set_parent 1/1 2
+wait
+assert_stacking 1/2 1/1
+
+local_activate 1/2
+assert_stacking 1/2 1/1

src/tests/test-client.c

@@ -136,6 +136,31 @@ process_line (const char *line)
         }
 
     }
+  else if (strcmp (argv[0], "set_parent") == 0)
+    {
+      if (argc != 3)
+        {
+          g_print ("usage: menu <window-id> <parent-id>");
+          goto out;
+        }
+
+      GtkWidget *window = lookup_window (argv[1]);
+      if (!window)
+        {
+          g_print ("unknown window %s", argv[1]);
+          goto out;
+        }
+
+      GtkWidget *parent_window = lookup_window (argv[2]);
+      if (!parent_window)
+        {
+          g_print ("unknown parent window %s", argv[2]);
+          goto out;
+        }
+
+      gtk_window_set_transient_for (GTK_WINDOW (window),
+                                    GTK_WINDOW (parent_window));
+    }
   else if (strcmp (argv[0], "show") == 0)
     {
       if (argc != 2)

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 ||