[issue15496] harden directory removal for tests on Windows

[Jeremy Kloth]

Jeremy Kloth added the comment:

> Tim Golden added the comment:
> I'm tentative about using ctypes this just because I don't believe we use it anywhere else.

ctypes is already used later in test_support so I figured it was fine
to use for this as well.

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<http://bugs.python.org/issue15496>
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