selftests: forwarding: forwarding.config.sample: Add port with no cable connected

Add NETIF_NO_CABLE port to tests topology.

The port can also be declared as an environment variable and tests can be
run like that:
NETIF_NO_CABLE=eth9 ./test.sh eth{1..8}

The NETIF_NO_CABLE port will be used by ethtool_extended_state test.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>