selftests/bpf: Make reference_tracking test use subtests

reference_tracking is actually a set of 9 sub-tests. Make it explicitly so.

Also, add explicit "classifier/" prefix to BPF program section names to
let libbpf correctly guess program type. Thus, also remove explicit
bpf_prog__set_type() call.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191021033902.3856966-7-andriin@fb.com