https://golang.org/cl/18087 added a bunch of t.Parallel calls, which
aren't compatible with the afterTest func. But in short mode, afterTest
is a no-op. To keep all.bash (short mode) fast, conditionally set
t.Parallel when in short mode, but keep it unset for compatibility with
afterFunc otherwise.

Fixes #13804

Change-Id: Ie841fbc2544e1ffbee43ba1afbe895774e290da0
Reviewed-on: https://go-review.googlesource.com/18143Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>