selftests/bpf: Add extra link to uprobe_multi tests

selftests/bpf: Add extra link to uprobe_multi tests

Attaching extra program to same functions system wide for api
and link tests.

This way we can test the pid filter works properly when there's
extra system wide consumer on the same uprobe that will trigger
the original uprobe handler.

We expect to have the same counts as before.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-29-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>