errgroup: fix build errors in errgroup_test Change-Id: Ie5ebfa26b6234f833139784da859d32cc1416b26 Reviewed-on: https://go-review.googlesource.com/24961 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>