selftests: mlxsw: Add VxLAN configuration test for IPv6

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

Add separated test cases for learning which is not supported for IPv6
and for UDP checksum flags which are different from IPv4 flags.

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