nixos/tests/gnome3-gdm: Increase memory limit

The test was failing on x86_64 prematurely due to memory being exhausted.

See also 3b9f0c6a469a574c1c030e4c23f6c766819cbd2d

nixos/tests/gnome3-gdm.nix

···
       };
       services.xserver.desktopManager.gnome3.enable = true;
 
-      virtualisation.memorySize = 512;
+      virtualisation.memorySize = 1024;
     };
 
   testScript =