selftests/bpf: Add uprobe_multi cookie test

Adding test for cookies setup/retrieval in uprobe_link uprobes
and making sure bpf_get_attach_cookie works properly.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-27-jolsa@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>