selftests: mlxsw: Add a test for VxLAN configuration

Test various aspects of VxLAN offloading which are specific to mlxsw,
such as sanitization of invalid configurations and offload indication.

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