selftests/bpf: Add start_test helper in bpf_tcp_ca

For moving the "if (sk_stg_map)" block out of do_test(), extract the
code before this block as a new function start_test(). It creates
server-side and client-side sockets and returns them to the caller.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/48f2921ff9be958f5d3d28fe6bb7269a61cafa9f.1717054461.git.tanggeliang@kylinos.cn