Skip the test requiring ctypes if ctypes is unavailable.

prevents http://buildbot.python.org/all/builders/x86%20Ubuntu%20Shared%203.x/builds/240/steps/test/logs/stdio

diff --cc Lib/test/test_subprocess.py
index d747845bc919d27d89b03ee790a874688b10ce4f,2dc03ee7001bf383d1d1e23c8d646c427da78c45..e63f9f254cd2b248451a3d3a6162716cdb1e8b3b
--- 1/Lib/test/test_subprocess.py
--- 2/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@@ -4,9 -5,9 +4,8 @@@ from test import suppor
  import subprocess
  import sys
  import platform
- import ctypes
  import signal
  import io
 -import locale
  import os
  import errno
  import tempfile