`support/arbitrary`: We should use `rand` in tests, not `arbitrary`.

*On 2022-10-28 @recmo wrote in [`49f6271`](https://github.com/recmo/uint/commit/49f6271d9967c55a71f588ed267387b57a285d43) “Merge pull request #195 from recmo/pow2”:*

We should use `rand` in tests, not `arbitrary`.

```rust

// TODO: Instead of uniform random sampling, we should use a distribution that
// exercises different scales more. Something like sum(±2ⁱ for random i). The
// reduction step can then remove terms or make them smaller.

// TODO: We should use `rand` in tests, not `arbitrary`.

impl<'a, const BITS: usize, const LIMBS: usize> Arbitrary<'a> for Uint {
fn arbitrary(u: &mut Unstructured<'a>) -> Result {
let mut limbs = [0; LIMBS];
if let Some((last, rest)) = limbs.split_last_mut() {

```
*From [`src/support/arbitrary.rs:11`](https://github.com/recmo/uint/blob/49f6271d9967c55a71f588ed267387b57a285d43/src/support/arbitrary.rs#L11)*