nixos/tests: fix Plasma 5 test

nixos/release-combined.nix

···
         (all nixos.tests.ecryptfs)
         (all nixos.tests.ipv6)
         (all nixos.tests.i3wm)
-        (all nixos.tests.kde5)
+        (all nixos.tests.plasma5)
         #(all nixos.tests.lightdm)
         (all nixos.tests.login)
         (all nixos.tests.misc)

nixos/release.nix

···
   tests.influxdb = callTest tests/influxdb.nix {};
   tests.ipv6 = callTest tests/ipv6.nix {};
   tests.jenkins = callTest tests/jenkins.nix {};
-  tests.kde5 = callTest tests/kde5.nix {};
+  tests.plasma5 = callTest tests/plasma5.nix {};
   tests.keymap = callSubTests tests/keymap.nix {};
   tests.initrdNetwork = callTest tests/initrd-network.nix {};
   tests.keystone = callTest tests/keystone.nix {};

nixos/tests/kde5.nix nixos/tests/plasma5.nix

···
         user = "alice";
       };
     };
-    services.xserver.desktopManager.kde5.enable = true;
+    services.xserver.desktopManager.plasma5.enable = true;
+    services.xserver.desktopManager.default = "plasma5";
     virtualisation.writableStore = false; # FIXME
   };