selftests: mlxsw: RED: Test offload of mirror on RED early_drop qevent

Add a selftest for offloading a mirror action attached to the block
associated with RED early_drop qevent.

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