path/filepath: fix TestEvalSymlinks when run under symlinked GOROOT
path/filepath: fix TestEvalSymlinks when run under symlinked GOROOT

Fixes issue 1622.

R=rsc, bradfitzgo
CC=golang-dev
http://codereview.appspot.com/4287063
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go