net: fix build fix

Now builders say:
pkg/net/dnsclient_unix_test.go:10: imported and not used: "runtime"

R=golang-dev
CC=golang-dev
https://golang.org/cl/12890043

src/pkg/net/dnsclient_unix_test.go

@@ -7,7 +7,6 @@
 package net
 
 import (
-	"runtime"
 	"testing"
 )