Found with staticcheck. Not terribly important since the test would
likely fail anyway, but at least it will fail with a better explanation
now.

Change-Id: Ic3f9a94a2152404b7873cc8cd47b6db79d78c2e6
Reviewed-on: https://go-review.googlesource.com/62990
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>