We will soon switch GO111MODULE to 'on' by default, and when that
happens these tests will otherwise break.

Updates #30228

Change-Id: I1016d429b1dfb889d1aae8bc86fb2567cf0fc56f
Reviewed-on: https://go-review.googlesource.com/c/162697
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>