tests: Move 'wait-for-X11-display' helper to MetaContextTest

Gets rid of a un-prefixed helper function relying on a global singleton.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/tests/unit-tests.c

@@ -19,6 +19,8 @@
 
 #include "config.h"
 
+#include "tests/unit-tests.h"
+
 #include <glib.h>
 #include <stdlib.h>
 
@@ -35,6 +37,8 @@
 #include "tests/monitor-transform-tests.h"
 #include "tests/wayland-unit-tests.h"
 
+MetaContext *test_context;
+
 typedef struct _MetaTestLaterOrderCallbackData
 {
   GMainLoop *loop; /* Loop to terminate when done. */
@@ -239,6 +243,8 @@ main (int argc, char *argv[])
                                       META_CONTEXT_TEST_FLAG_TEST_CLIENT);
   g_assert (meta_context_configure (context, &argc, &argv, NULL));
 
+  test_context = context;
+
   init_tests ();
 
   g_signal_connect (context, "before-tests",