Add this timer test

[ry]

const timers = [];
let timeouts = 0;

for (let i = 0; i < 10; i++) {
  timers[i] = setTimeout(onTimeout, 20);
}

function onTimeout() {
  ++timeouts;
  for (let i = 1; i < 10; i++) {
    clearTimeout(timers[i]);
  }
}

assert(timeouts === 1);

[hayd]

At the moment this is non-deterministic. I get timeouts as either 2 or 1 (on a 2 core machine).