selftests/bpf: Add a test case for unix sockmap

Add a test case to ensure redirection between two AF_UNIX
datagram sockets work.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210704190252.11866-11-xiyou.wangcong@gmail.com