Only run recipe simulation tests if infra/ changes.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index a3af9e4445b46bc28c05c97698b93d8b28929d42..e0e01fb30fd7b3918b1c184a5a57be4c9072704b 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -172,6 +172,10 @@ def _ToolFlags(input_api, output_api):
 def _RecipeSimulationTest(input_api, output_api):
   """Run the recipe simulation test."""
   results = []
+  if not any(f.LocalPath().startswith('infra')
+             for f in input_api.AffectedFiles()):
+    return results
+
   recipes_py = os.path.join('infra', 'bots', 'recipes.py')
   cmd = ['python', recipes_py, 'simulation_test']
   try: