selftests/bpf: Add libbpf feature-probing API selftests

selftests/bpf: Add libbpf feature-probing API selftests

Add selftests for prog/map/prog+helper feature probing APIs. Prog and
map selftests are designed in such a way that they will always test all
the possible prog/map types, based on running kernel's vmlinux BTF enum
definition. This way we'll always be sure that when adding new BPF
program types or map types, libbpf will be always updated accordingly to
be able to feature-detect them.

BPF prog_helper selftest will have to be manually extended with
interesting and important prog+helper combinations, it's easy, but can't
be completely automated.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/bpf/20211217171202.3352835-3-andrii@kernel.org