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

tv42/old-go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go