selftests/bpf: Add asserts for netfilter link info

Add assertions/tests to verify `bpf_link_info` fields for netfilter link
are correctly populated.

Signed-off-by: Tyrone Wu <wudevelops@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20241011193252.178997-2-wudevelops@gmail.com