selftests/bpf: Use start_server_str in test_tcp_check_syncookie_user

Since start_server_str() is added now, it can be used in script
test_tcp_check_syncookie_user.c instead of start_server_addr() to
simplify the code.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/5d2f442261d37cff16c1f1b21a2b188508ab67fa.1718932493.git.tanggeliang@kylinos.cn


Signed-off-by: Alexei Starovoitov <ast@kernel.org>