First cut of testing infrastructure for recipes.

scripts/slave/recipes/gatekeeper.py

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Specifies how to launch the gatekeeper."""
 
 def GetFactoryProperties(api, _factory_properties, build_properties):
   steps = api.Steps(build_properties)
   return {
     'steps': [
       steps.step('gatekeeper_launch',
-                 api.build_path('scripts', 'slave', 'gatekeeper_launch.py'))
+                 [api.build_path('scripts', 'slave', 'gatekeeper_launch.py')])
     ]
   }