misc/cgo/test: relax the threshold in test3250.

Fixes build for some slow FreeBSD/NetBSD/Darwin builder.

R=golang-dev
CC=golang-dev
https://golang.org/cl/11207043

misc/cgo/test/issue3250.go

@@ -46,7 +46,7 @@ import (
 
 func test3250(t *testing.T) {
 	const (
-		thres = 5
+		thres = 1
 		sig   = syscall.SIGCHLD
 	)
 	type result struct {