Add ability to freeze timeline clock for testing

Players may have non-zero start time, and/or playback rates other than
one. As noted in https://github.com/web-animations/web-animations-js/issues/628, calling document.timeline._pauseAnimationsForTesting 
with the current timeline time will change such players' currentTime, and
will change the animated elements' computed style.

We implement a new testing method _freezeClockForTesting that avoids
changing any of the players, it simply freezes the timeline's
currentTime to the supplied time.