test: make goprint.go wait longer for go its routine to execute

Update issue 5281

R=golang-dev, r, bradfitz, cshapiro
CC=golang-dev
https://codereview.appspot.com/8631047
testing/iotest: rephrase DataErrReader's description

R=r
CC=golang-dev
https://codereview.appspot.com/8650044

Committer: Rob Pike <r@golang.org>