selftests/bpf: Add test for bpf_override_return helper

selftests/bpf: Add test for bpf_override_return helper

We do not actually test the bpf_override_return helper functionality
itself at the moment, only the bpf program being able to attach it.

Adding test that override prctl syscall return value on top of
kprobe and kprobe.multi.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20260112121157.854473-2-jolsa@kernel.org