Memory leak when task contains handle to node it runs on

[m-mueller678]

This snippet leaks memory, which can be observed by running it in a loop. I think dropping the Runtime should cancel all tasks, which would presumably fix this.

let rt = madsim::runtime::Runtime::new();
let handle = rt.handle();
rt.block_on(async move {
    let node = handle.create_node().build();
    node.spawn({
        let node = node.clone();
        async move {
            madsim::time::sleep(std::time::Duration::from_secs(1)).await;
            drop(node);
        }
    });
});