net/http/cgi: correctly handle pathnames for cygwin perl on windows

Cygwin perl uses unix pathnames in windows. Include cygwin perl in the
list of special cases for unix pathname handling in test.cgi.

Change-Id: I30445a9cc79d62d022ecc232c35aa5015b7418dc
Reviewed-on: https://go-review.googlesource.com/2973


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>