bpf, sockmap: Add af_unix test with both sockets in map

This adds a test where both pairs of a af_unix paired socket are put into a
BPF map. This ensures that when we tear down the af_unix pair we don't have
any issues on sockmap side with ordering and reference counting.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20231129012557.95371-3-john.fastabend@gmail.com