nixos/plasma5: re-enable test

Fixes #24240.

I do not know the cause of the test failures, but the tests pass now.

nixos/release-combined.nix

@@ -72,7 +72,7 @@ in rec {
         (all nixos.tests.ecryptfs)
         (all nixos.tests.ipv6)
         (all nixos.tests.i3wm)
-        #(all nixos.tests.plasma5) # temporarily, see #24240
+        (all nixos.tests.plasma5)
         #(all nixos.tests.lightdm)
         (all nixos.tests.login)
         (all nixos.tests.misc)