This fixes the build for long-running tests after CL 139338

Change-Id: Ib8adfa785d41c736188e2ff7e14125de045b96b9
Reviewed-on: https://go-review.googlesource.com/c/139637
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>