sync/atomic: fix data race in tests Fixes issue 2710.
sync/atomic: fix data race in tests
Fixes issue 2710.

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/5541066

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