selftests/bpf: Add uprobe_multi bench test

Adding test that attaches 50k uprobes in uprobe_multi binary.

After the attach is done we run the binary and make sure we
get proper amount of hits.

The resulting attach/detach times on my setup:

test_bench_attach_uprobe:PASS:uprobe_multi__open 0 nsec
test_bench_attach_uprobe:PASS:uprobe_multi__attach 0 nsec
test_bench_attach_uprobe:PASS:uprobes_count 0 nsec
test_bench_attach_uprobe: attached in 0.346s
test_bench_attach_uprobe: detached in 0.419s
#262/5 uprobe_multi_test/bench_uprobe:OK

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-24-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>