selftests/bpf: Use start_test in test_dctcp_fallback in bpf_tcp_ca

The newly added helper start_test() can be used in test_dctcp_fallback()
too, to replace start_server_str() and connect_to_fd_opts(). In that
way, two network_helper_opts srv_opts and cli_opts are used instead of
the previously shared opts.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/792ca3bb013fa06e618176da02d75e4f79a76733.1717054461.git.tanggeliang@kylinos.cn