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

nixos/tests/common/user-account.nix

@@ -0,0 +1,11 @@
+{ pkgs, ... }:
+
+{ users.extraUsers = pkgs.lib.singleton
+    { name = "alice";
+      description = "Alice Foobar";
+      home = "/home/alice";
+      createHome = true;
+      useDefaultShell = true;
+      password = "foobar";
+    };
+}

nixos/tests/common/x11.nix

@@ -0,0 +1,12 @@
+{ services.xserver.enable = true;
+
+  # Automatically log in.
+  services.xserver.displayManager.auto.enable = true;
+
+  # Use IceWM as the window manager.
+  services.xserver.windowManager.default = "icewm";
+  services.xserver.windowManager.icewm.enable = true;
+
+  # Don't use a desktop manager.
+  services.xserver.desktopManager.default = "none";
+}