tests: Override X11 display number

As with the Wayland display name, to avoid clashes with already an
running Xwayland or Xorg instance, override the X11 display name to
something less likely to cause a clash.

https://gitlab.gnome.org/GNOME/mutter/issues/193

src/tests/test-utils.c

@@ -27,6 +27,7 @@
 #include "core/display-private.h"
 #include "core/window-private.h"
 #include "wayland/meta-wayland.h"
+#include "wayland/meta-xwayland.h"
 #include "x11/meta-x11-display-private.h"
 
 struct _TestClient {
@@ -94,6 +95,7 @@ test_init (int    *argc,
   ensure_test_client_path (*argc, *argv);
 
   meta_wayland_override_display_name ("mutter-test-display");
+  meta_xwayland_override_display_number (512);
 }
 
 AsyncWaiter *