Address testing guide nits

Fixes #19861 

/cc @huonw 
Oh, whoops, forgot to write it down:

```rust
    b.iter(|| {
        let mut n = 1000_u32;
        test::black_box(&mut n); // pretend to modify `n`
        range(0, n).fold(0, |a, b| a ^ b)
    })
```