selftests/bpf: validate STACK_ZERO is preserved on subreg spill

Add tests validating that STACK_ZERO slots are preserved when slot is
partially overwritten with subregister spill.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-6-andrii@kernel.org


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