nixos: add sddm test to combined constituents

Changed files
+1
nixos
+1
nixos/release-combined.nix
···
(all nixos.tests.openssh)
(all nixos.tests.printing)
(all nixos.tests.proxy)
+
(all nixos.tests.sddm)
(all nixos.tests.simple)
(all nixos.tests.udisks2)
(all nixos.tests.xfce)