KDE test: Bump kdm start timeout

Hopefully this will fix random failures like
http://hydra.nixos.org/build/36249079.

nixos/modules/testing/test-instrumentation.nix

···
 
     services.xserver.displayManager.logToJournal = true;
 
+    # Bump kdm's X server start timeout to account for heavily loaded
+    # VM host systems.
+    services.xserver.displayManager.kdm.extraConfig =
+      ''
+        [X-:*-Core]
+        ServerTimeout=240
+      '';
+
   };
 
 }