sync/atomic: do not run TestStoreLoadSeq for too long (fix windows builder)
sync/atomic: do not run TestStoreLoadSeq for too long (fix windows builder)

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/4962057

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go