Change error message of a failing `assert_eq!`

If ~1/3 of the uses choose one order, and the other ~2/3 use the other order, I don't think that's enough of a consensus for us to codify. It would be incorrect a lot of the time. Also, in my experience, when a programming language does choose an order for expected vs actual, my personal experience is getting this wrong a lot of the time (and seeing others get it wrong a lot of the time). So I think rust's approach of not choosing a canonical ordering for this is better.