selftests: devlink_trap: Add test cases for devlink-trap

Add test cases for devlink-trap on top of the netdevsim implementation.

The tests focus on the devlink-trap core infrastructure and user space
API. They test both good and bad flows and also dismantle of the netdev
and devlink device used to report trapped packets.

This allows device drivers to focus their tests on device-specific
functionality.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>