selftests: mlxsw: Add Q-in-VNI veto tests

Add tests to ensure that the forbidden and unsupported cases are indeed
vetoed by mlxsw driver.

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