selftests/bpf: Introduce socket_helpers.h for TC tests

Pull socket helpers out of sockmap_helpers.h so that they can be reused
for TC tests as well. This prepares for the next patch.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241213034057.246437-4-xiyou.wangcong@gmail.com