selftests/bpf: Test low-level perf BPF link API

Add tests utilizing low-level bpf_link_create() API to create perf BPF link.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210815070609.987780-13-andrii@kernel.org