cmd/go/testdata/script: remove skips and clarify comments in mod_*_upgrade_pseudo tests
These tests were added in CL 174206.
They required a 'git' binary and network access in an earlier draft,
but now use the test-local module proxy instead, so no longer need to
be skipped when those resources are not present.
Updates #30634
Change-Id: I5f36c6c776209a89bc45d133847df5052b55da59
Reviewed-on: https://go-review.googlesource.com/c/go/+/176537
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>