os: check only user attributes in TestStatDirModeExec
os: check only user attributes in TestStatDirModeExec

Some have their system setup in a particular way,
see http://golang.org/issue/4444#c3.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/6851129

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