net, os, file/filepath, syscall: use slices.Equal in tests

net, os, file/filepath, syscall: use slices.Equal in tests

Use slices.Equal to compare slices instead of strings.Join and then
comparing strings.

Change-Id: Ib916002b7357bd7f4e66b853dd7af8d98eba5549
Reviewed-on: https://go-review.googlesource.com/c/go/+/690475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>