selftests/bpf: Test bpf_skc_to_unix_sock() helper

Add a new test which triggers unix_listen kernel function
to test bpf_skc_to_unix_sock helper.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211021134752.1223426-3-hengqi.chen@gmail.com