selftests: seg6: add selftest for SRv6 End.DT46 Behavior

this selftest is designed for evaluating the new SRv6 End.DT46 Behavior
used, in this example, for implementing IPv4/IPv6 L3 VPN use cases.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Paolo Lungaroni <paolo.lungaroni@uniroma2.it>
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>