selftests/bpf: Add kprobe_multi bpf_cookie test

Adding bpf_cookie test for programs attached by kprobe_multi links.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220316122419.933957-12-jolsa@kernel.org