tests(clan): move test-inventory.nix to clanLib. And name it 'makeTestClan'

Exposing the function via clanLib makes it more accessible to clan modules
This allows each module to define its own test without needing to depend on any fileSystem path assumptions in the clan-core repo

checks/flake-module.nix

@@ -27,9 +27,10 @@ in
         let
           nixosTestArgs = {
             # reference to nixpkgs for the current system
-            inherit pkgs;
+            inherit pkgs lib;
             # this gives us a reference to our flake but also all flake inputs
             inherit self;
+            inherit (self) clanLib;
           };
           nixosTests = lib.optionalAttrs (pkgs.stdenv.isLinux) {
             # import our test