test: don't remove empty.txt on win32

on win32 we use empty.txt in the fixtures directory, otherwise we
use a file constructed specifically for this test due to POSIX
socket path length limitations, in which case we need to do
appropriate cleanup

Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/iojs/io.js/pull/2