selftests: mlxsw: RED: Test offload of trapping on RED qevents

Add a selftest for RED early_drop and mark qevents when a trap action is
attached at the associated block.

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