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>