Submission/unipro web animations 05

Tests for AnimationTimeline,play() method
This seems to implement current-time-of-a-player (https://web-animations.github.io/web-animations-spec/bikeshed/Overview.html#the-current-time-of-a-player), but the spec is a bit more elaborate, for example it will return the `hold time` if it is resolved.

As the `hold time` is not exposed I think it's sufficient for relevant tests to directly encode the expected result of the computation.

I think there will be ongoing changes in the spec here, what is the best way to track this? For this kind of issue I think we can land the test if it's in sync with what the spec says. For spec changes after the test lands we will come back and update the tests.