selftests: add selftest for the SRv6 End.DT4 behavior

this selftest is designed for evaluating the new SRv6 End.DT4 behavior
used, in this example, for implementing IPv4 L3 VPN use cases.

Signed-off-by: Andrea Mayer <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

Author: skorpion17