raw_io: fix passing retcode in test_api.stream_output

recipe_modules/raw_io/test_api.py

 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 from recipe_engine import recipe_test_api
 
 class RawIOTestApi(recipe_test_api.RecipeTestApi): # pragma: no cover
   @recipe_test_api.placeholder_step_data
   @staticmethod
   def output(data, retcode=None, name=None):
     return data, retcode, name
 
   @recipe_test_api.placeholder_step_data
   @staticmethod
   def output_dir(files_dict, retcode=None, name=None):
     assert type(files_dict) is dict
     assert all(type(key) is str for key in files_dict.keys())
     assert all(type(value) is str for value in files_dict.values())
     return files_dict, retcode, name
 
   def stream_output(self, data, stream='stdout', retcode=None, name=None):
     ret = recipe_test_api.StepTestData()
     assert stream in ('stdout', 'stderr')
     step_data = self.output(data, retcode=retcode, name=name)
     setattr(ret, stream, step_data.unwrap_placeholder())
+    if retcode:
+      ret.retcode = retcode
     return ret