os: use SameFile in TestStartProcess Fixes issue 4650.
os: use SameFile in TestStartProcess
Fixes issue 4650.

R=golang-dev, bradfitz, alex.brainman
CC=golang-dev
https://codereview.appspot.com/7085048

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang