Check for stray threads from tests and fail if found

testing_support/auto_stub.py

Index: testing_support/auto_stub.py
diff --git a/testing_support/auto_stub.py b/testing_support/auto_stub.py
index e1155a64ae71f4ec5e0607ad95dd2eb352dd14c8..bcede8773a416835d1d8b5373201ac302af3a82e 100644
--- a/testing_support/auto_stub.py
+++ b/testing_support/auto_stub.py
@@ -4,7 +4,8 @@
 
 import collections
 import inspect
-import unittest
+
+from testing_support import thread_watcher
 
 
 class AutoStubMixIn(object):
@@ -63,8 +64,8 @@ class SimpleMock(object):
         caller_name, ', '.join(filter(None, [str_args, str_kwargs]))))
 
 
-class TestCase(unittest.TestCase, AutoStubMixIn):
+class TestCase(thread_watcher.TestCase, AutoStubMixIn):
   """Adds self.mock() and self.has_failed() to a TestCase."""
   def tearDown(self):
     AutoStubMixIn.tearDown(self)
-    unittest.TestCase.tearDown(self)
+    super(TestCase, self).tearDown()