Tests for AnimationNode attributes

Tests for AnimationNode attributes
Should also test the case with groups that can effect computed timing, for example:

new AnimationSequence([
  new Animation(element, [...], 100), // computedTiming.fill == 'forwards', computedTiming.startTime == 0
  new Animation(element, [...], 100) // computedTiming.fill == 'forwards', computedTiming.startTime == 100
], {fill: 'forwards'});

However if these types of tests are being added later, we are ok with this being in another file.