Move NixOS systems tests to `nixos/tests`.

This mirrors the organization of upstream NixOS tests, and hopefully makes them easier to find.

nixos/system-tests.nix

@@ -2,6 +2,6 @@
 let
   pkgs = import ../nixpkgs-2105.nix { };
 in {
-  private-storage = pkgs.nixosTest ./modules/tests/private-storage.nix;
-  tahoe = pkgs.nixosTest ./modules/tests/tahoe.nix;
+  private-storage = pkgs.nixosTest ./tests/private-storage.nix;
+  tahoe = pkgs.nixosTest ./tests/tahoe.nix;
 }