tests/wayland: Move test client helper into a utils file

This will make it easier to run Wayland test cases from other test
executables.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2262>

src/tests/meta-wayland-test-utils.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2021-2022 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef META_WAYLAND_TEST_UTILS_H
+#define META_WAYLAND_TEST_UTILS_H
+
+typedef struct _MetaWaylandTestClient MetaWaylandTestClient;
+
+MetaWaylandTestClient * meta_wayland_test_client_new (const char *test_client_name);
+
+void meta_wayland_test_client_finish (MetaWaylandTestClient *wayland_test_client);
+
+#endif /* META_WAYLAND_TEST_UTILS_H */