path/filepath: fix test
path/filepath: fix test

Make it possible to run test over symlinked GOROOT.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6268046
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go