selftests/bpf: Add bpf_program__attach_netfilter helper test

Call bpf_program__attach_netfilter() with different
protocol/hook/priority combinations.

Test fails if supposedly-illegal attachments work
(e.g., bogus protocol family, illegal priority and so on) or if a
should-work attachment fails. Expected output:

./test_progs -t netfilter_link_attach
#145/1 netfilter_link_attach/allzero:OK
#145/2 netfilter_link_attach/invalid-pf:OK
#145/3 netfilter_link_attach/invalid-hooknum:OK
#145/4 netfilter_link_attach/invalid-priority-min:OK
#145/5 netfilter_link_attach/invalid-priority-max:OK
#145/6 netfilter_link_attach/invalid-flags:OK
#145/7 netfilter_link_attach/invalid-inet-not-supported:OK
#145/8 netfilter_link_attach/attach ipv4:OK
#145/9 netfilter_link_attach/attach ipv6:OK

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/bpf/20230628152738.22765-3-fw@strlen.de