Function invoke_sub_vm_expect 

pub fn invoke_sub_vm_expect(
    nub: &Nub,
    top: &SubVmTop,
    depth: u64,
    expected_return: u64,
)

Like invoke_sub_vm but also asserts the top-level return value. Used by the data-recurse correctness check to confirm each level reads its pinned RO data + writes its initial RW data correctly (not just that it halts).