This should make parallel execution a bit clearer.
With -p=1 it should make the execution completely unambiguous.

Fixes #19280.

Change-Id: Ib48cdfe96896d01b0d8f98ccb2fab614407a7d92
Reviewed-on: https://go-review.googlesource.com/49430
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>