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

In a similar fashion to the asymmetric test, add a test for symmetric
routing. In symmetric routing both the ingress and egress VTEPs perform
routing in the overlay network into / from the VxLAN tunnel. Packets in
different directions use the same VNI - the L3 VNI.
Different tenants (VRFs) use different L3 VNIs.

Add a test which is similar to the existing IPv4 test to check IPv6.
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>