Change the test API.

  test_at(x.x, function() {})
changes to
  timing_test(function() {
     at(x.x, function() {});
  }

There also exists a new at command which should be used instead of setTimeout
or testharness_timeline.schedule.

Using at inside a normal testharness test() function is considered an error.
Mainly automatic transform of test_at to the timing_test(function() { at() }) API.
Merge pull request #270 from mithro/test-api-fix

Mainly automatic transform of test_at to the timing_test(function() { at() }) API.
Test that styles and timing are visible after a player seek
Repeat the last tick at last tick time when active animation state changes.

This includes changes to:

 * Players which have ticked at least once.
 * The animation tree (including changing the source of a Player)
 * Specified timings

A repeat is not triggered by changes to AnimationEffects.
Rename Animation.animationEffect to effect and use a bool in exitModifyCurrentAnimationState
Make depth counting exception safe
Test chaning player start time
Use timed_test API