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

nixos/tests/kexec.nix

@@ -0,0 +1,18 @@
+# Test whether fast reboots via kexec work.
+
+{ pkgs, ... }:
+
+{
+
+  machine = { config, pkgs, ... }:
+    { virtualisation.vlans = [ ]; };
+
+  testScript =
+    ''
+      $machine->waitForUnit("multi-user.target");
+      $machine->execute("systemctl kexec &");
+      $machine->{connected} = 0;
+      $machine->waitForUnit("multi-user.target");
+    '';
+
+}