selftests: forwarding: Add router test with VID 1

Previous patches made it possible to setup VLAN devices with VID 1 over
mlxsw ports. Verify this functionality actually works by conducting a
simple router test over VID 1.

Adding this test as a generic test since it can be run using veth pairs
and it can also be useful for other physical devices where VID 1 was
considered reserved (knowingly or not).

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>