selftests/bpf: Use start_server in bpf_tcp_ca

To simplify the code, use BPF selftests helper start_server() in
bpf_tcp_ca.c instead of open-coding it. This helper is defined in
network_helpers.c, and exported in network_helpers.h, which is already
included in bpf_tcp_ca.c.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/9926a79118db27dd6d91c4854db011c599cabd0e.1711331517.git.tanggeliang@kylinos.cn