selftests: netdevsim: Extend devlink trap test to include flow action cookie

Extend existing devlink trap test to include metadata type for flow
action cookie.

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