selftests: forwarding: Add VxLAN tests with a VLAN-aware bridge for IPv6

The tests are very similar to their VLAN-unaware counterpart
(vxlan_bridge_1d_ipv6.sh and vxlan_bridge_1d_port_8472_ipv6.sh),
but instead of using multiple VLAN-unaware bridges, a single VLAN-aware
bridge is used with multiple VLANs.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>