selftests: add selftest for the SRv6 End.DX6 behavior with netfilter

this selftest is designed for evaluating the SRv6 End.DX6 behavior
used with netfilter(rpfilter), in this example, for implementing
IPv6 L3 VPN use cases.

Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>