Prevent spurious leaks when running regrtest.py -R. There may be more

issues that crop up from time to time, but this change seems to have been
pretty stable (no spurious warnings) for about a week.

Other modules which use threads may require similar use of
threading_setup/threading_cleanup from test_support.

Lib/test/test_socket.py

@@ -906,7 +906,10 @@ def test_main():
         tests.append(BasicSocketPairTest)
     if sys.platform == 'linux2':
         tests.append(TestLinuxAbstractNamespace)
+
+    thread_info = test_support.threading_setup()
     test_support.run_unittest(*tests)
+    test_support.threading_cleanup(*thread_info)
 
 if __name__ == "__main__":
     test_main()