According to issue #19314 waitid on Darwin returns if the process is
stopped, even though we specify WEXITED.

Fixes #19314.

Change-Id: I95faf196c11e43b7741efff79351bab45c811bc2
Reviewed-on: https://go-review.googlesource.com/37610
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>