misc/cgo/testso: use bash to run test.bash

R=golang-dev, minux.ma, capnm9
CC=golang-dev
https://golang.org/cl/9717043

misc/cgo/testso/test.bash

-#!/bin/sh
+#!/usr/bin/env bash
 # Copyright 2011 The Go Authors.  All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.