selftests/bpf: Add test for bpf_get_link_xdp_id

Add xdp_info selftest that makes sure that bpf_get_link_xdp_id returns
valid prog_id for different input modes:

  * w/ and w/o flags when no program is attached;
  * w/ and w/o flags when one program is attached.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/2a9a6d1ce33b91ccc1aa3de6dba2d309f2062811.1586236080.git.rdna@fb.com