selftests/bpf: add a test case for vsock sockmap

Add a test case testing the redirection from connectible AF_VSOCK
sockets to connectible AF_UNIX sockets.

Signed-off-by: Bobby Eshleman <bobby.eshleman@bytedance.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>