function disableTrigger(app, callback, done) {
  var origElement = app.$element;
  app.$element = function() { return $('.doesNotExist'); };
  callback();
  app.$element = origElement;
  done();
}
Disables the Sammy trigger mechanism by
setting an invalid element as $element.

This is for example necessary when triggering
errors, because mocha is going to catch those
and display them as errors, even if we catch
the original error with throwException().