cmd/dist: add support for disabling test -short mode
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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,akutz/go,golang/go,cloudflare/go,josharian/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,golang/go,josharian/go,nawawi/go,akutz/go,golang/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go,nawawi/go,josharian/go,josharian/go,cloudflare/go,nawawi/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,josharian/go,josharian/go