selftest/bpf: Add tests for sockmap with unix stream type.

Add two tests for unix stream to unix stream redirection
in sockmap tests.

Signed-off-by: Jiang Wang <jiang.wang@bytedance.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210816190327.2739291-4-jiang.wang@bytedance.com