selftests/bpf: Use connect_to_fd in test_tcp_check_syncookie

selftests/bpf: Use connect_to_fd in test_tcp_check_syncookie

This patch uses public helper connect_to_fd() exported in network_helpers.h
instead of the local defined function connect_to_server() in
test_tcp_check_syncookie_user.c. This can avoid duplicate code.

Then the arguments "addr" and "len" of run_test() become useless, drop them
too.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/e0ae6b790ac0abc7193aadfb2660c8c9eb0fe1f0.1714907662.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>