Address testing guide nits

Fixes #19861 

/cc @huonw 
This benchmark is still incorrect: the text implies that this new benchmark is then fixed (i.e. accurately benchmarking the time it takes to xor 1000 `uint`s), but it's not. It looks like we may need a new benchmark, or stronger black-boxing:

```rust
let mut n = 1000;
// `n` is unchanged, butthe optimiser is forced to assume that `n` may be mutated
test::black_box(&mut n);

b.iter(|| {
    test::black_box(range(0u, n).fold(...));
})
```
(or the version that does the `range.fold` without a semicolon.)