selftests: mlxsw: Add one-armed router test

Construct a "one-armed router" topology and test that packets are
forwarded by the ASIC and that a copy of the packet is sent to the
kernel, which does not forward the packet again.

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