Add support for parallel presubmit unit testing.

PRESUBMIT.py

 # Copyright (c) 2012 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.
 
 """Top-level presubmit script for depot tools.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into depot_tools.
 """
 
 
 def CommonChecks(input_api, output_api, tests_to_black_list):
   results = []
   results.extend(input_api.canned_checks.CheckOwners(input_api, output_api))
   black_list = list(input_api.DEFAULT_BLACK_LIST) + [
       r'^cpplint\.py$',
       r'^cpplint_chromium\.py$',
       r'^python_bin[\/\\].+',
       r'^site-packages-py[0-9]\.[0-9][\/\\].+',
       r'^svn_bin[\/\\].+',
       r'^testing_support[\/\\]_rietveld[\/\\].+']
   disabled_warnings = [
     'R0401',  # Cyclic import
     'W0613',  # Unused argument
   ]
-  results.extend(input_api.canned_checks.RunPylint(
-      input_api,
-      output_api,
-      white_list=[r'.*\.py$'],
-      black_list=black_list,
-      disabled_warnings=disabled_warnings))
-
-  # TODO(maruel): Make sure at least one file is modified first.
-  # TODO(maruel): If only tests are modified, only run them.
-  results.extend(input_api.canned_checks.RunUnitTestsInDirectory(
-      input_api,
-      output_api,
-      'tests',
-      whitelist=[r'.*test\.py$'],
-      blacklist=tests_to_black_list))
+  results.extend(input_api.RunTests(
+      input_api.canned_checks.GetPylint(
+          input_api,
+          output_api,
+          white_list=[r'.*\.py$'],
+          black_list=black_list,
+          disabled_warnings=disabled_warnings) +
+      # TODO(maruel): Make sure at least one file is modified first.
+      # TODO(maruel): If only tests are modified, only run them.
+      input_api.canned_checks.GetUnitTestsInDirectory(
+          input_api,
+          output_api,
+          'tests',
+          whitelist=[r'.*test\.py$'],
+          blacklist=tests_to_black_list)))
   return results
 
 
 def RunGitClTests(input_api, output_api):
   """Run all the shells scripts in the directory test.
   """
   if input_api.platform == 'win32':
     # Skip for now as long as the test scripts are bash scripts.
     return []
 
@@ skipping 51 matching lines @@
 
 
 def CheckChangeOnCommit(input_api, output_api):
   output = []
   output.extend(CommonChecks(input_api, output_api, []))
   output.extend(input_api.canned_checks.CheckDoNotSubmit(
       input_api,
       output_api))
   output.extend(RunGitClTests(input_api, output_api))
   return output