[issue16444] Use support.TESTFN_UNDECODABLE on UNIX

[STINNER Victor]

STINNER Victor added the comment:

> Please test the patch on UNIX, Windows and Mac OS X.

The full test suite pass on:

 * Linux with UTF-8 locale encoding
 * Linux with ASCII locale encoding
 * Windows with cp932 ANSI code page
 * Mac OS 10.8 with ASCII locale encoding (and utf-8/surrogateescape for the filesystem encoding) ($LANG, $LC_ALL, $LC_CTYPE are not set)

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16444>
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