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

scripts/slave/recipe_modules/path/test_api.py

Index: scripts/slave/recipe_modules/path/test_api.py
diff --git a/scripts/slave/recipe_modules/path/test_api.py b/scripts/slave/recipe_modules/path/test_api.py
index 5e80bfa09a4a8419ab2c55484e17898c97af0ef4..c966feb7e53d4e791e40f26399f061cf9fa68920 100644
--- a/scripts/slave/recipe_modules/path/test_api.py
+++ b/scripts/slave/recipe_modules/path/test_api.py
@@ -10,8 +10,3 @@ class PathTestApi(recipe_test_api.RecipeTestApi):
 
   def __getitem__(self, name):
     return Path(NamedBasePath(name))
-
-  def listdir(self, files):
-    def listdir_callback():
-      return self.m.json.output(files)
-    return listdir_callback