selftests/bpf: Update xdp_bpf2bpf test to use new set_attach_target API

Use the new bpf_program__set_attach_target() API in the xdp_bpf2bpf
selftest so it can be referenced as an example on how to use it.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/158220520562.127661.14289388017034825841.stgit@xdp-tutorial