Add e2e tests for boolean checkbox in page data form

Pushed the required fix here.

The nuance is this in FormBuilder::handleInputElement

// Get the input for the current element. NULL values in the input need
        // to be explicitly distinguished from missing input. (see below)
        $input_exists = NULL;
        $input = NestedArray::getValue($form_state->getUserInput(), $element['#parents'], $input_exists);
        // For browser-submitted forms, the submitted values do not contain
        // values for certain elements (empty multiple select, unchecked
        // checkbox). During initial form processing, we add explicit NULL
        // values for such elements in FormState::$input. When rebuilding the
        // form, we can distinguish elements having NULL input from elements
        // that were not part of the initially submitted form and can therefore
        // use default values for the latter, if required. Programmatically
        // submitted forms can submit explicit NULL values when calling
        // self::submitForm() so we do not modify FormState::$input for them.
        if (!$input_exists && !$form_state->isRebuilding() && !$form_state->isProgrammed()) { // 👈️👈️👈️👈️👈️👈️👈️
          // Add the necessary parent keys to FormState::$input and sets the
          // element's input value to NULL.
          NestedArray::setValue($form_state->getUserInput(), $element['#parents'], NULL);
          $input_exists = TRUE;
        }

With \Drupal\experience_builder\ClientDataToEntityConverter::setEntityFields we're doing a $form_state->setProgrammed() so that set value never happens. Therefore the old code that was unsetting (or rather filtering out) unticked checkboxes actually needs to be setting them to NULL rather than removing them.

With that change in place, the test passes 🙌

Rebased, addressed review comments and #9