selftests: forwarding: add a no_forwarding.sh test

Bombard a standalone switch port with various kinds of traffic to ensure
it is really standalone and doesn't leak packets to other switch ports.
Also check for switch ports in different bridges, and switch ports in a
VLAN-aware bridge but having different pvids. No forwarding should take
place in either case.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>