[json] improve output to get leak_to and better logging.

recipe_modules/json/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.
 
 import json
 
 from recipe_engine import recipe_test_api
 
 class JsonTestApi(recipe_test_api.RecipeTestApi):
   @recipe_test_api.placeholder_step_data
   @staticmethod
   def output(data, retcode=None, name=None):
+    """Supplies placeholder data for a json.output. `data` should be a jsonish
+    python object (e.g. dict, list, str, bool, int, etc). It will be dumped out
+    with json.dumps and the step will be observed to return that dumped value.
+    """
     return json.dumps(data), retcode, name
 
+  def invalid(self, raw_data_str, retcode=None, name=None):
+    """Can be used to supply data for a json.output, except that it takes a raw
+    string rather than a json object."""
+    ret = recipe_test_api.StepTestData()
+    ret.retcode=retcode
+    placeholder_data = recipe_test_api.PlaceholderTestData(
+      data=raw_data_str, name=name)
+    ret.placeholder_data[(self._module.NAME, 'output', name)] = placeholder_data
+    return ret
+
   def output_stream(self, data, stream='stdout', retcode=None, name=None):
     assert stream in ('stdout', 'stderr')
     ret = recipe_test_api.StepTestData()
     step_data = self.output(data, retcode=retcode, name=name)
     setattr(ret, stream, step_data.unwrap_placeholder())
     return ret