Create test harness

Add a simple test harness for running layout tests.

Currently, the tests are run manually by opening test pages in the browser and checking the console for PASSED or FAILED reports.

Initial tests exercise the following animation element types: animate, animateTransform and set.

We create an animation using the Polyfill implementation, and another
using the native implementation, and verify property expectations at a
sequence of times.

Unfortunately, the animated value of the fill property
isn't being reported for the natively animated element, only for the
element animated using the SMIL Polyfill. (With the other properties
being animated, this problem isn't occurring.) We animate the fill value
in color.html and attempt to test in color-check.js