Refactor the way that TestApi works so that it is actually useful.

scripts/slave/recipe_modules/generator_script/api.py

Index: scripts/slave/recipe_modules/generator_script/api.py
diff --git a/scripts/slave/recipe_modules/generator_script/api.py b/scripts/slave/recipe_modules/generator_script/api.py
index e5ed6c896c040beca36a1385b65f2066dc07cfc6..fd172dbcd2dc28d55230a62ecf394d03fc9912e7 100644
--- a/scripts/slave/recipe_modules/generator_script/api.py
+++ b/scripts/slave/recipe_modules/generator_script/api.py
@@ -7,16 +7,17 @@ from slave import recipe_api
 class GeneratorScriptApi(recipe_api.RecipeApi):
   def __call__(self, path_to_script, *args):  # pragma: no cover
     """Run a script and generate the steps emitted by that script."""
+    f = '--output-json'
     step_name = 'gen step(%s)' % self.m.path.basename(path_to_script)
     if path_to_script.endswith('.py'):
       yield self.m.python(
         step_name,
-        path_to_script, list(args) + [self.m.json.output()],
+        path_to_script, list(args) + [f, self.m.json.output()],
         cwd=self.m.path.checkout())
     else:
       yield self.m.step(
         step_name,
-        [path_to_script,] + list(args) + [self.m.json.output()],
+        [path_to_script,] + list(args) + [f, self.m.json.output()],
         cwd=self.m.path.checkout())
     new_steps = self.m.step_history.last_step().json.output
     assert isinstance(new_steps, list)