selftest/bpf: Add new tests in sockmap for unix stream to tcp.

Add two new test cases in sockmap tests, where unix stream is
redirected to tcp and vice versa.

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: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210816190327.2739291-6-jiang.wang@bytedance.com