selftests: add selftest for the SRv6 End.DT6 (VRF) behavior

this selftest is designed for evaluating the new SRv6 End.DT6 (VRF) behavior
used, in this example, for implementing IPv6 L3 VPN use cases.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Paolo Lungaroni <paolo.lungaroni@cnit.it>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>