selftests: mlxsw: Add test cases for devlink-trap policers

Add test cases that verify that each registered packet trap policer:

* Honors that imposed limitations of rate and burst size
* Able to police trapped packets to the specified rate
* Able to police trapped packets to the specified burst size
* Able to be unbound from its trap group

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