selftests/bpf: Pass BPF skeleton to sockmap_listen ops tests

Following patch extends the sockmap ops tests to cover the scenario when a
sockmap with attached programs holds listening sockets.

Pass the BPF skeleton to sockmap ops test so that the can access and attach
the BPF programs.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20230113-sockmap-fix-v2-3-1e0ee7ac2f90@cloudflare.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>