imp: add some provenance info on defaults

modules/testing/default.nix

@@ -53,6 +53,7 @@ in
             description = "Default to apply to test";
             type = types.unspecified;
             default = { };
+            apply = default: { _file = "testing.defaults"; } // default;
           };
         };
       }));

modules/testing/docker.nix

@@ -40,6 +40,7 @@ in
   config.testing.defaults = [{
     features = [ "docker" ];
     default = {
+      _file = "testing.docker.registryUrl";
       docker.registry.url = cfg.registryUrl;
     };
   }];