cmd/go: test 'go get' and 'go install' with an (invalid) module ending in /v1

cmd/go: test 'go get' and 'go install' with an (invalid) module ending in /v1

We expect major version v1 to not have a /v1 suffix.
(Such a suffix on the package path is fine, but not on the module path.)

Fixes #26375

Change-Id: I4206ded82ede36440cedfaf39875c38b9c95dc27
Reviewed-on: https://go-review.googlesource.com/c/153823
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>