selftests/bpf: Add test cases for redirection between udp and unix

Add two test cases to ensure redirection between udp and unix
work bidirectionally.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210704190252.11866-12-xiyou.wangcong@gmail.com