selftests/bpf: Allow for iteration over multiple ports

Prepare to test TCP socket iteration over both listening and established
sockets by allowing the BPF iterator programs to skip the port check.

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>