selftests: forwarding: Add Q-in-VNI test for IPv6

Add test to check Q-in-VNI traffic with IPv6 underlay and overlay.
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: Jakub Kicinski <kuba@kernel.org>