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
Merge pull request #2 from iojs/test-net-pipe-connect-errors-socket-path-win-fix-2

test: don't remove empty.txt on win32