doc/codewalk: test source code

Fixes #2648.

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

src/run.bash

diff --git a/src/run.bash b/src/run.bash
index 4d8d04b..24bcd3f 100755
--- a/src/run.bash
+++ b/src/run.bash
@@ -152,10 +152,7 @@
 ) || 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