selftests: forwarding: Add a test for VxLAN asymmetric routing with IPv6

In asymmetric routing the ingress VTEP routes the packet into the
correct VxLAN tunnel, whereas the egress VTEP only bridges the packet to
the correct host. Therefore, packets in different directions use
different VNIs - the target VNI.

Add a test which is similar to the existing IPv4 test to check IPv6.

The test uses a simple topology with two VTEPs and two VNIs and verifies
that ping passes between hosts (local / remote) in the same VLAN (VNI)
and in different VLANs belonging to the same tenant (VRF).

While the test does not check VM mobility, it does configure an anycast
gateway using a macvlan device on both VTEPs.
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>