selftests: nic_link_layer: Add selftest case for speed and duplex states

Add selftest case for testing the speed and duplex state of
local NIC driver and the partner based on the supported
link modes obtained from the ethtool. Speed and duplex states
are varied and verified using ethtool.

Signed-off-by: Mohan Prasad J <mohan.prasad@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>