ResourceTestbase does not respect $uniqueFieldNames when duplicating the test entity

Problem/Motivation

The ResourceTestBase has a static property of $uniqueFieldNames for tracking unique field names on an entity. This is used to prevent errors when modifying and creating entities in tests.

These fields are not taken into account when the \Drupal\Tests\jsonapi\Functional\ResourceTestBase::getEntityDuplicate method is used.

I had to extend this method to fix the problem, where the sku field is unique.

  /**
   * {@inheritdoc}
   */
  protected function getEntityDuplicate(EntityInterface $original, $key) {
    $dupe = parent::getEntityDuplicate($original, $key);
    assert($dupe instanceof ProductVariationInterface);
    $dupe->setSku('XYZ789');
    return $dupe;
  }

Steps to reproduce

See #3128322: Filtering variations over JSON:API is always access false

Proposed resolution

Respect unique fields when making a duplicate. It is done there:

  protected function getModifiedEntityForPostTesting() {
    $document = $this->getPostDocument();

    // Ensure that all the unique fields of the entity type get a new random
    // value.
    foreach (static::$uniqueFieldNames as $field_name) {
      $field_definition = $this->entity->getFieldDefinition($field_name);
      $field_type_class = $field_definition->getItemDefinition()->getClass();
      $document['data']['attributes'][$field_name] = $field_type_class::generateSampleValue($field_definition);
    }

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet