Test harness does not allow timing_test()s to be started after page has finished loading

[steveblock]

This functionality would be useful for tests where we need to wait for some asynchronous set-up, eg MediaReference tests ...

async_test(function(testObject) {
  waitForSomethingAsync(function() {
    timing_test(function() {
      at(1.0, function() {
        assert_true(...);
        testObject.done();
      });
    });
  });
});