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

lib/default.nix

@@ -25,6 +25,7 @@ lib.fix (clanLib: {
   buildClanModule = clanLib.callLib ./build-clan { };
   inventory = clanLib.callLib ./inventory { };
   modules = clanLib.callLib ./inventory/frontmatter { };
+  test = clanLib.callLib ./tests { };
 
   # Plain imports.
   values = import ./introspection { inherit lib; };