selftests: seg6: add selftest for PSP flavor in SRv6 End behavior

This selftest is designed for testing the PSP flavor in SRv6 End behavior.
It instantiates a virtual network composed of several nodes: hosts and
SRv6 routers. Each node is realized using a network namespace that is
properly interconnected to others through veth pairs.
The test makes use of the SRv6 End behavior and of the PSP flavor needed
for removing the SRH from the IPv6 header at the penultimate node.

The correct execution of the behavior is verified through reachability
tests carried out between hosts.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Paolo Lungaroni <paolo.lungaroni@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>