selftests: mlxsw: Add test cases for devlink-trap layer 3 drops

Test that each supported packet trap is triggered under the right
conditions and that packets are indeed dropped and not forwarded.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>