selftests: forwarding: Add tests for ethtool extended state

Add tests to check ethtool report about extended state.
The tests configure several states and verify that the correct extended
state is reported by ethtool.

Check extended state with substate (Autoneg) and extended state without
substate (No cable).

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>