runtime/debug: fix test when source cannot be found
runtime/debug: fix test when source cannot be found

This happens with GOROOT_FINAL=/somewhere/else

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5727069
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go