selftests: forwarding: Add speed and auto-negotiation test

Check configurations and packets transference with different variations
of autoneg and speed.

Test plan:
1. Test force of same speed with autoneg off
2. Test force of different speeds with autoneg off (should fail)
3. One side is autoneg on and other side sets force of common speeds
4. One side is autoneg on and other side only advertises a subset of the
   common speeds (one speed of the subset)
5. One side is autoneg on and other side only advertises a subset of the
   common speeds. Check that highest speed is negotiated
6. Test autoneg on, but each side advertises different speeds (should
   fail)

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