nixos/release-combined: re-enable lightdm test

This has been tested in the Pantheon test
for a year now and it does fine on hydra.

nixos/release-combined.nix

@@ -104,7 +104,7 @@ in rec {
         #(all nixos.tests.keymap.neo)
         #(all nixos.tests.keymap.qwertz)
         (all nixos.tests.plasma5)
-        #(all nixos.tests.lightdm)
+        (all nixos.tests.lightdm)
         (all nixos.tests.login)
         (all nixos.tests.misc)
         (all nixos.tests.mutableUsers)

nixos/tests/all-tests.nix

@@ -139,7 +139,7 @@ in
   ldap = handleTest ./ldap.nix {};
   leaps = handleTest ./leaps.nix {};
   lidarr = handleTest ./lidarr.nix {};
-  #lightdm = handleTest ./lightdm.nix {};
+  lightdm = handleTest ./lightdm.nix {};
   limesurvey = handleTest ./limesurvey.nix {};
   login = handleTest ./login.nix {};
   loki = handleTest ./loki.nix {};