selftests/net: Add env for container based tests

selftests/net: Add env for container based tests

Add an env NetDrvContEnv for container based selftests. This automates
the setup of a netns, netkit pair with one inside the netns, and a BPF
program that forwards skbs from the NETIF host inside the container.

Currently only netkit is used, but other virtual netdevs e.g. veth can
be used too.

Expect netkit container datapath selftests to have a publicly routable
IP prefix to assign to netkit in a container, such that packets will
land on eth0. The BPF skb forward program will then forward such packets
from the host netns to the container netns.

Signed-off-by: David Wei <dw@davidwei.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/20260305181803.2912736-4-dw@davidwei.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>