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.