selftests/bpf: Create established sockets in socket iterator tests

Prepare for bucket resume tests for established TCP sockets by creating
established sockets. Collect socket fds from connect() and accept()
sides and pass them to test cases.

Signed-off-by: Jordan Rife <jordan@jrife.io>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>