unix: add test for Getdirentries
unix: add test for Getdirentries

Copied and adapted from the respective test in std syscall.

Change-Id: Ia35864e201c8b628dfc21a8da79305d5741d4acc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/183219
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Yuval Pavel Zholkover <170f3b94ed768c88b460d7f0c362c1d8ffaf0cec@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

multiplay/golang-sys,golang/sys,golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys