syscall: disable TestPassFD on openbsd

Update #4956.

R=golang-dev
CC=golang-dev
https://golang.org/cl/7417048

src/pkg/syscall/passfd_test.go

@@ -13,6 +13,7 @@ import (
 	"net"
 	"os"
 	"os/exec"
+	"runtime"
 	"syscall"
 	"testing"
 	"time"
@@ -26,6 +27,9 @@ import (
 // "-test.run=^TestPassFD$" and an environment variable used to signal
 // that the test should become the child process instead.
 func TestPassFD(t *testing.T) {
+	if runtime.GOOS == "openbsd" {
+		t.Skip("issue 4956")
+	}
 	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
 		passFDChild()
 		return