unix: test nonblocking and close-on-exec behavior of Pipe2
unix: test nonblocking and close-on-exec behavior of Pipe2

Following TestNonblockingPipe in package runtime.

Change-Id: Ifacec3ea6ce372a891273e3ec9fa4194f7d17619
Reviewed-on: https://go-review.googlesource.com/c/sys/+/295869
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,golang/sys,golang/sys