doc/codewalk: test source code

Fixes #2648.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/11331043

doc/codewalk/run

+#!/usr/bin/env bash
+# Copyright 2013 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.
+
+set -e
+
+function fail {
+	echo FAIL: doc/codewalk/$1
+	exit 1
+}
+
+# markov.xml
+echo foo | go run markov.go | grep foo > /dev/null || fail markov
+
+# functions.xml
+go run pig.go | grep 'Wins, losses staying at k = 100: 210/990 (21.2%), 780/990 (78.8%)' > /dev/null || fail pig
+
+# sharemem.xml: only build the example, as it uses the network
+go build -o /dev/null urlpoll.go || fail urlpoll
+

src/run.bash

@@ -152,10 +152,7 @@ make clean || exit 1
 ) || exit $?
 
 (xcd ../doc/codewalk
-# TODO: test these too.
-go build pig.go || exit 1
-go build urlpoll.go || exit 1
-rm -f pig urlpoll
+time ./run || exit 1
 ) || exit $?
 
 echo