selftests: net: use netdevsim in netns test

Netdevsim has extra register_netdevice_notifier_dev_net notifiers,
use netdevim instead of dummy device to test them out.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250401163452.622454-9-sdf@fomichev.me


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