selftests/bpf: Use start_server_addr in sk_assign

selftests/bpf: Use start_server_addr in sk_assign

Include network_helpers.h in prog_tests/sk_assign.c, use the newly
added public helper start_server_addr() instead of the local defined
function start_server(). This can avoid duplicate code.

The code that sets SO_RCVTIMEO timeout as timeo_sec (3s) can be dropped,
since start_server_addr() sets default timeout as 3s.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/2af706ffbad63b4f7eaf93a426ed1076eadf1a05.1713427236.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>