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

Add tests similar to vxlan_bridge_1d.sh and vxlan_bridge_1d_port_8472.sh.

The tests set up a topology with three VxLAN endpoints: one
"local", possibly offloaded, and two "remote", formed using veth pairs
and likely purely software bridges. The "local" endpoint is connected to
host systems by a VLAN-unaware bridge.

Since VxLAN tunnels must be unique per namespace, each of the "remote"
endpoints is in its own namespace. H3 forms the bridge between the three
domains.

Send IPv4 packets and IPv6 packets with IPv6 underlay.
Use `TC_FLAG`, which is defined in `forwarding.config` file, for TC
checks. `TC_FLAG` allows testing that on HW datapath, the traffic
actually goes through HW.

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