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>
1 file changed