Remove waitAndTest from html imports tests

The waitAndTest system was flaky, it was busy waiting on a condition to
become true using requestAnimationFrame. We should just use the load
events on the links instead. This simplifies the tests and makes them
more stable.

R=morrita@chromium.org

Review URL: https://codereview.chromium.org/299533002

git-svn-id: svn://svn.chromium.org/blink/trunk@174331 bbb929c8-8fbe-4397-9dbb-9b2b20218538