selftests: forwarding: Add a test for flooded traffic

Add test cases for unknown unicast and unregistered multicast flooding.

For each traffic type, turn off flooding on one bridged port and inject
a packet of the specified type through the second bridged port. Make
sure the packet was not received by checking the ACL counters on the
other end. Later, turn on flooding and make sure the packet was
received.

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