Add a await spec.pause(i) helper

[jalada]

Sometimes you just want your spec to pause or slow down, maybe to visually confirm something, or because it's too fiddly to use exists to wait for something to happen.

const pause = function(i) {
  const p = new Promise((resolve, reject) => {
    setTimeout(function() {
      resolve();
    }, i);
  });

  return p;
}