syscall: disable TestPassFD on openbsd
syscall: disable TestPassFD on openbsd

Update issue 4956.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7417048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go