path.TestWalk: disable error case if root (chmod 0 doesn't cause errors for root)
path.TestWalk: disable error case if root
(chmod 0 doesn't cause errors for root)

Fixes issue 22.

R=gri
http://codereview.appspot.com/152120

wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old