bpf: adding tests for map_in_map helpber in libbpf

adding test/example of bpf_map__set_inner_map_fd usage

Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>