Add test for WeekTableElement in a form (not a widget)

Problem/Motivation

In ✨ Use the office hours field widget as Element (in a settings form) Fixed , the Weektable was separated into a widget and an element.
This calls for a test, that makes sure the functionality is preserved.

Steps to reproduce

Create a for like this:

 /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form = [];

    $config = \Drupal::configFactory()->get('example.settings');
    $field_settings = OfficeHoursItem::defaultStorageSettings();
    $widget_settings = OfficeHoursWeekWidget::defaultSettings();
    $office_hours = $config->get('office_hours') ?? [];

    $form['office_hours'] = [
      '#type' => 'office_hours_table',
      '#title' => $this->t('Office hours'),
      '#default_value' => $office_hours,

      '#field_settings' => $field_settings,
      '#widget_settings' => $widget_settings,
      '#field_type' => $week_season_id = 0,
    ];

    $form['actions'] = [
      '#type' => 'actions',
      'submit' => [
        '#type' => 'submit',
        '#value' => $this->t('Save'),
      ],
    ];

    return $form;
  }

And test if the submitted values are preserved the next time the form is displayed.

Proposed resolution

As per #23 in above issue, for the ECA module, some form submission tests exist and you can find an example at https://git.drupalcode.org/project/eca/-/blob/2.0.x/modules/form/tests/s...

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