main: Add test initialization function

Since Clutter's backend relies on MetaBackend now, initialzation has
to go through meta_init(), both in mutter and in gnome-shell.

However the compositor enum and backend gtype used to enforce the
environment used for tests are private, so instead expose a test
initialization function that can be used from both mutter and
gnome-shell.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/750

src/tests/clutter-test-utils.h

@@ -86,13 +86,7 @@ G_BEGIN_DECLS
 int \
 main (int argc, char *argv[]) \
 { \
-  char *display_name = g_strdup ("mutter-test-display-XXXXXX");\
-  int fd = g_mkstemp (display_name);\
-  meta_wayland_override_display_name (display_name);\
-  meta_override_compositor_configuration (META_COMPOSITOR_TYPE_WAYLAND,\
-                                          META_TYPE_BACKEND_X11_NESTED);\
-\
-  meta_init ();\
+  meta_test_init ();\
 \
   clutter_test_init (&argc, &argv); \
 \
@@ -100,8 +94,6 @@ main (int argc, char *argv[]) \
     units \
   } \
 \
-  close (fd);\
-  g_free (display_name);\
   return clutter_test_run (); \
 }