clan tests: convert makeTestClan to a module

Let's not put yet another wrapper around runTest, instead expose our logic as a module that can be imported into any nixos-vm-test

clanServices/hello-world/flake-module.nix

@@ -47,8 +47,8 @@ in
           hello-service = import ./tests/vm/default.nix {
             inherit module;
             inherit self inputs pkgs;
-            # clanLib is exposed from inputs.clan-core
-            clanLib = self.clanLib;
+            nixosLib = import (self.inputs.nixpkgs + "/nixos/lib") { };
+            clan-core = self;
           };
         };
     };