When -test.failfast flag is provided to go test,
no new tests get started after the first failure.

Fixes #21700

Change-Id: I0092e72f25847af05e7c8e1b811dcbb65a00cbe7
Reviewed-on: https://go-review.googlesource.com/74450
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>