AnimationPlayer.ready will currently always be resolved but by updating these
tests to wait on it anyway they should continue to work once we introduce the
delayed animation start behavior.
This patch moves commonly used addDiv and waitForFrame test methods to
a separate testcommon.js support file.
It also takes advantage of the updates to testharness.js from bug 1104433 to
rework addDiv such that it automatically removes the created div at the end of
the test.
