Otherwise, if the test is run in the background, it will stop waiting
for access to the terminal.

Change-Id: Ib5224c6cb9060281e05c3b00cd2964445421e774
Reviewed-on: https://go-review.googlesource.com/c/136415
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>