selftests/bpf: Add uprobe_multi link test

Adding uprobe_multi test for bpf_link_create attach function.

Testing attachment using the struct bpf_link_create_opts.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-22-jolsa@kernel.org


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