cmd/go: improve TestScript/reuse_git

cmd/go: improve TestScript/reuse_git

Most reuse origin info in git should not list RepoSum; check that.

Also add tests of go list -reuse, which were omitted entirely
in CL 411398.

Also be extra sure there is no repo left in module cache during reuse tests.

For #75119.

Change-Id: Ia1436b8d9e17db49664d24f1d43c448ac7cd00d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/718380
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>