unix: use slices.{Equal,Sort} in tests

Same as CL 587655 and CL 690475 did for package syscall tests.

Change-Id: Ie3c8726a4e2c859b5d7992ea1baec14083b9fdba
Reviewed-on: https://go-review.googlesource.com/c/sys/+/708558
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>