selftests/bpf: add selftest validating callback result is enforced

selftests/bpf: add selftest validating callback result is enforced

BPF verifier expects callback subprogs to return values from specified
range (typically [0, 1]). This requires that r0 at exit is both precise
(because we rely on specific value range) and is marked as read
(otherwise state comparison will ignore such register as unimportant).

Add a simple test that validates that all these conditions are enforced.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231202175705.885270-6-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>