Merge branch 'Use start_server and connect_fd_to_fd'

Geliang Tang says:

====================
Simplify bpf_tcp_ca test by using connect_fd_to_fd and start_server
helpers.

v4:
 - Matt reminded me that I shouldn't send a square-to patch to BPF (thanks),
   so I update them into two patches in v4.

v3:
 - split v2 as two patches as Daniel suggested.
 - The patch "selftests/bpf: Use start_server in bpf_tcp_ca" is merged
   by Daniel (thanks), but I forgot to drop 'settimeo(lfd, 0)' in it, so
   I send a squash-to patch to fix this.
====================

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