selftests: mlxsw: Add a test for locked port trap

Test that packets received via a locked bridge port whose {SMAC, VID}
does not appear in the bridge's FDB or appears with a different port,
trigger the "locked_port" packet trap.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>