ui_test parallelism does not work as expected

[RalfJung]

I was wondering why tests seem to execute so sequentially. I think I figured it out: each of our N threads runs while let Some(path) = todo.pop() {; that means if the queue is ever empty when they look at it, the thread terminates and does not come back!

I think we might want to use rayon with its thread pool, rather than trying an ad-hoc implementation of a threadpool ourselves.