Move all of NixOS to nixos/ in preparation of the repository merge

nixos/tests/xfce.nix

@@ -0,0 +1,32 @@
+{ pkgs, ... }:
+
+{
+
+  machine =
+    { config, pkgs, ... }:
+
+    { imports = [ ./common/user-account.nix ];
+
+      services.xserver.enable = true;
+
+      services.xserver.displayManager.auto.enable = true;
+      services.xserver.displayManager.auto.user = "alice";
+
+      services.xserver.desktopManager.xfce.enable = true;
+    };
+
+  testScript =
+    ''
+      $machine->waitForWindow(qr/xfce4-panel/);
+      $machine->sleep(10);
+
+      # Check that logging in has given the user ownership of devices.
+      $machine->succeed("getfacl /dev/snd/timer | grep -q alice");
+
+      $machine->succeed("su - alice -c 'DISPLAY=:0.0 xfce4-terminal &'");
+      $machine->waitForWindow(qr/Terminal/);
+      $machine->sleep(10);
+      $machine->screenshot("screen");
+    '';
+
+}