Fix depot_tools presubmit tests by mocking multiprocessing.cpu_count.

tests/presubmit_unittest.py

Index: tests/presubmit_unittest.py
diff --git a/tests/presubmit_unittest.py b/tests/presubmit_unittest.py
index f37bf6ca6b3c14ec38e74f9ac3026dd6f311b8f3..3bec7733c92ffaca5fd5396e927a7b716d34eae0 100755
--- a/tests/presubmit_unittest.py
+++ b/tests/presubmit_unittest.py
@@ -7,11 +7,12 @@
 
 # pylint: disable=E1101,E1103
 
+import StringIO
 import functools
 import itertools
 import logging
+import multiprocessing
 import os
-import StringIO
 import sys
 import time
 import unittest
@@ -156,6 +157,10 @@ def GetPreferredTryMasters(project, change):
     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)."""