Force zero startTime for animation in test-duration.html

This avoids spurious test failures where the animation uses a non-zero
startTime due to polyfill start-up time.
Trivial style fixes to a couple of tests
Clean up test-playback-rate.html and test-pause.html
Move test code to a new test/ directory

This will keep the directory structure clean when new test infrastructure is
added.
Merge pull request #70 from steveblock/testing

Testing