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/2973Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>