selftests/bpf: Add kprobe_multi kmod attach api tests

Adding kprobe_multi kmod attach api tests that attach bpf_testmod
functions via bpf_program__attach_kprobe_multi_opts.

Running it as serial test, because we don't want other tests to
reload bpf_testmod while it's running.

Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20221025134148.3300700-9-jolsa@kernel.org


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