selftests: mlxsw: Add VxLAN FDB veto test for IPv6

Add test to verify FDB vetos of VxLAN with IPv6 underlay.
Use the existing test which checks IPv4.

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