cmd/go: add go list -test to describe test binaries

cmd/go: add go list -test to describe test binaries

Tools should be able to ask cmd/go about the dependency
graph for test binaries instead of reinventing it themselves.
Allow them to do so, with the new list -test flag.

This also fixes and tests for a bug introduced in CL 104315
that was not properly splitting dependencies on the path
between package main and the package being tested.

Change-Id: I29eb454c82893f5ee70252aaaecd9fa376eaf3c8
Reviewed-on: https://go-review.googlesource.com/107916
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>