[release-branch.go1.12] cmd/go/internal/modfetch: update TestCodeRepo for gopkg.in/yaml.v2 again

Update the expected data to fix the longtest builder.

Updates #28856

Change-Id: I7fb6ee72e8469d974561b4b4057f40142f5b3654
Reviewed-on: https://go-review.googlesource.com/c/go/+/198557


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
(cherry picked from commit 64785bf9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/198700
Run-TryBot: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 17a492fd)
Reviewed-on: https://go-review.googlesource.com/c/go/+/200038