selftests: mlxsw: Add test for VxLAN related traps for IPv6

The test configures VxLAN with IPv6 underlay and verifies that the
expected traps are triggered under the right conditions.

The test is similar to the existing IPv4 test.

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