selftests/net: Add netkit container tests

selftests/net: Add netkit container tests

Add two tests using NetDrvContEnv. One basic test that sets up a netkit
pair, with one end in a netns. Use LOCAL_PREFIX_V6 and nk_forward BPF
program to ping from a remote host to the netkit in netns.

Second is a selftest for netkit queue leasing, using io_uring zero copy
test binary inside of a netns with netkit. This checks that memory
providers can be bound against virtual queues in a netkit within a
netns that are leasing from a physical netdev in the default netns.

Signed-off-by: David Wei <dw@davidwei.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260115082603.219152-17-daniel@iogearbox.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>