cmd/dist: add support for disabling test -short mode

So we can have builders running go test -short=false.

Updates golang/go#12508

Change-Id: If90f0f6d9f89268c33b1d1876139ad551fecd3d8
Reviewed-on: https://go-review.googlesource.com/113435


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>