cmd/go/internal/modfetch: update TestCodeRepo for gopkg.in/yaml.v2 v2.2.2

I think we ought to make these tests hermetic, but in the meantime we
should at least make them pass.

Fixes #27692
Updates #28856

Change-Id: Ia78fa60e998dea3c871f640ffa2ece67b054f866
Reviewed-on: https://go-review.googlesource.com/c/153460


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>