build: lengthen timeout for the lengthy runtime test

        To fix build on (overheated) Linux/ARM builder.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/5865051

src/run.bash

@@ -29,7 +29,7 @@ time go test std -short -timeout=120s
 echo
 
 echo '# GOMAXPROCS=2 runtime -cpu=1,2,4'
-GOMAXPROCS=2 go test runtime -short -timeout=120s -cpu=1,2,4
+GOMAXPROCS=2 go test runtime -short -timeout=240s -cpu=1,2,4
 echo
 
 echo '# sync -cpu=10'