Extract functions from path recipe module so that step can depend on it

scripts/slave/recipe_modules/path/test_api.py

 from slave import recipe_test_api
 from slave.recipe_config_types import Path, NamedBasePath
 
 class PathTestApi(recipe_test_api.RecipeTestApi):
   @recipe_test_api.mod_test_data
   @staticmethod
   def exists(*paths):
     assert all(isinstance(p, Path) for p in paths)
     return paths
 
   def __getitem__(self, name):
     return Path(NamedBasePath(name))
-
-  def listdir(self, files):
-    def listdir_callback():
-      return self.m.json.output(files)
-    return listdir_callback