Fixes this test on s390x.

Change-Id: Ie5b70e8191169867765ec9248d827ca12c6405f4
Reviewed-on: https://go-review.googlesource.com/20964Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>