Fix depot_tools presubmit tests by mocking multiprocessing.cpu_count.

tests/presubmit_unittest.py

 #!/usr/bin/env python
 # 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.
 
 """Unit tests for presubmit_support.py and presubmit_canned_checks.py."""
 
 # pylint: disable=E1101,E1103
 
+import StringIO
 import functools
 import itertools
 import logging
+import multiprocessing
 import os
-import StringIO
 import sys
 import time
 import unittest
 
 _ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.insert(0, _ROOT)
 
 from testing_support.super_mox import mox, SuperMoxTestBase
 
 import owners
@@ skipping 124 matching lines @@
     presubmit.os.getcwd = self.RootDir
     presubmit.os.chdir = MockChdir
     self.mox.StubOutWithMock(presubmit.scm, 'determine_scm')
     self.mox.StubOutWithMock(presubmit.scm.SVN, '_CaptureInfo')
     self.mox.StubOutWithMock(presubmit.scm.SVN, 'GetFileProperty')
     self.mox.StubOutWithMock(presubmit.gclient_utils, 'FileRead')
     self.mox.StubOutWithMock(presubmit.gclient_utils, 'FileWrite')
     self.mox.StubOutWithMock(presubmit.scm.SVN, 'GenerateDiff')
     self.mox.StubOutWithMock(presubmit.scm.GIT, 'GenerateDiff')
 
+    # On some platforms this does all sorts of undesirable system calls, so
+    # just permanently mock it with a lambda that returns 2
+    multiprocessing.cpu_count = lambda: 2
+
 
 class PresubmitUnittest(PresubmitTestsBase):
   """General presubmit_support.py tests (excluding InputApi and OutputApi)."""
 
   _INHERIT_SETTINGS = 'inherit-review-settings-ok'
 
   def testMembersChanged(self):
     self.mox.ReplayAll()
     members = [
       'AffectedFile', 'Change', 'DoGetTrySlaves',
@@ skipping 2725 matching lines @@
         owners_check=False)
     self.assertEqual(1, len(results))
     self.assertEqual(
         'Found line ending with white spaces in:', results[0]._message)
     self.checkstdout('')
 
 
 if __name__ == '__main__':
   import unittest
   unittest.main()