strconv: add tests for issue 2917
strconv: add tests for issue 2917

Cannot reproduce the failure locally,
but add explicit test in case some other
machine can.

Fixes issue 2917 (for now).

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5651071

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