Stabilize FunctionalJavascript testing AJAX: make ::setValue() trigger both "input" and "formUpdated" events

Problem/Motivation

In #3313946: Update CKEditor 5 to 35.2.1, a work-around had to be introduced because \Behat\Mink\Driver\Selenium2Driver::setValue() does:

        // Remove the focus from the element if the field still has focus in
        // order to trigger the change event. By doing this instead of simply
        // triggering the change event for the given xpath we ensure that the
        // change event will not be triggered twice for the same element if it
        // has lost focus in the meanwhile. If the element has lost focus
        // already then there is nothing to do as this will already have caused
        // the triggering of the change event for that element.
        $script = <<<JS
var node = {{ELEMENT}};
if (document.activeElement === node) {
  document.activeElement.blur();
}
JS;

        // Cover case, when an element was removed from DOM after its value was
        // changed (e.g. by a JavaScript of a SPA) and therefore can't be focused.
        try {
            $this->executeJsOnElement($element, $script);
        } catch (StaleElementReference $e) {
            // Do nothing because an element was already removed and therefore
            // blurring is not needed.
        }

… but blurring is not always a desired event. Really, Selenium should've been triggering the input event instead.

Related, in #3316274: Stabilize FunctionalJavascript testing AJAX: add ::assertExpectedAjaxRequest() it was discovered that calling ::setValue() may not trigger the necessary AJAX system updates, or at least not in time, because core/misc/form.js does:

        // Initialize form behaviors, use $.makeArray to be able to use native
        // forEach array method and have the callback parameters in the right
        // order.
        $.makeArray($forms).forEach((form) => {
          const events = 'change.formUpdated input.formUpdated ';
          const eventHandler = debounce((event) => {
            triggerFormUpdated(event.target);
          }, 300);
          formFields = fieldsList(form).join(',');

          form.setAttribute('data-drupal-form-fields', formFields);
          $(form).on(events, eventHandler);
        });

In other words: it listens to the input event, which never gets triggered, and it debounces it to allow updates at most every 300 milliseconds. But in tests there is no need for debouncing, since there is no continuous typing at all. In other words: it'd be safe to not debounce; which we can simulate by triggering formUpdated immediately, which would in turn slightly speed up functional JS tests. This part is optional though.

Steps to reproduce

Proposed resolution

Override ::setValue()'s behavior.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet