cmd/go: add a script test for artifacts resulting from 'go get -u'

For #36460

Change-Id: I4f8bf0fb8dfa508b346acb3868302452409ee9da
Reviewed-on: https://go-review.googlesource.com/c/go/+/289696


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>