selftests: seg6: add selftest for SRv6 H.L2Encaps.Red behavior

This selftest is designed for testing the H.L2Encaps.Red 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 considers SRv6 routers implementing a L2 VPN leveraged by hosts
for communicating with each other. Such routers make use of the SRv6
H.L2Encaps.Red behavior for applying SRv6 policies to L2 traffic coming
from hosts.

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

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: David S. Miller <davem@davemloft.net>