cmd/go: test that 'go get pkg@version' installs pkg

This test passes, but it encodes several behaviors that I think are bugs.
I suggest that we check it in as-is, and we can update it as the bugs are fixed.

Change-Id: Icb073de9cb13036dbccadb4ff2cb3169ffb56236
Reviewed-on: https://go-review.googlesource.com/128137


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>