cmd/go/internal/modfetch: update TestCodeRepo for gopkg.in/yaml.v2

New release, it broke again the longtest builder. Align the expected
data with the current release.

Making these tests independent of external repositories is #28856.

This fixes the longtest builder.

Updates #28856

Change-Id: I32d2f3325af828d26ab417a5e986e3aeefa1a897
Reviewed-on: https://go-review.googlesource.com/c/go/+/168561


Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>