Merge branch 'use network helpers, part 5'

Geliang Tang says:

====================
This patchset uses post_socket_cb callbacks of struct network_helper_opts
to refactor do_test() in bpf_tcp_ca.c.

v5:
 - address Martin's comments in v4 (thanks)
 - add patch 4, use start_server_str in test_dctcp_fallback too
 - ASSERT_* is already used in settcpca, use this helper in cc_cb (patch 3).

v4:
 - address Martin's comments in v3 (thanks).
 - drop 2 patches, keep "type" as the individual arg to start_server_addr,
   connect_to_addr and start_server_str.

v3:
 - Add 4 new patches, 1-3 are cleanups. 4 adds a new helper.
 - address Martin's comments in v2.

v2:
 - rebased on commit "selftests/bpf: Add test for the use of new args in
 cong_control"
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>