selftests: net: use Path helpers in ping

Now that net and net-next have converged we can use the Path
helpers in the ping test without conflicts.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250327222315.1098596-4-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>