selftests: devlink_trap_tunnel_ipip: Add test case for decap_error

Test that the trap is triggered under the right conditions and that
devlink counters increase.

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>