nixos/tests/gnome3: select X11 gnome shell explicitely

This isn't strictly necessary yet as LightDM doesn't read the wayland sessions,
but there's no harm in being explicit.

nixos/tests/gnome3.nix

@@ -16,7 +16,7 @@ import ./make-test.nix ({ pkgs, ...} : {
       services.xserver.displayManager.lightdm.autoLogin.enable = true;
       services.xserver.displayManager.lightdm.autoLogin.user = "alice";
       services.xserver.desktopManager.gnome3.enable = true;
-      services.xserver.desktopManager.default = "gnome";
+      services.xserver.desktopManager.default = "gnome-xorg";
 
       virtualisation.memorySize = 1024;
     };