[release-branch.go1.13] os: re-enable TestPipeThreads on darwin

CL 197938 actually fixes those regression on Darwin as syscalls
are no longer labeled as always blocking and consume a thread.

Fixes #34712

Change-Id: I82c98516c23cd36f762bc5433d7b71ea8939a0ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/199477


Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit cfe23204)
Reviewed-on: https://go-review.googlesource.com/c/go/+/200105


Run-TryBot: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>