class ViewsTestEntity

class ViewsTestEntity extends ContentEntityBase {
  
  /**
   * The mocked base fields for test entity types.
   *
   * An array keyed by entity type ID, whose values are arrays of base field
   * definitions.
   *
   * @var array
   */
  protected static $mockedBaseFieldDefinitions = [];
  
  /**
   * Sets up the mocked base field definitions.
   *
   * @param string $entity_type_id
   *   The entity type ID.
   * @param array $definitions
   *   The array of base field definitions to mock. These are added to the
   *   defaults ones from the parent class.
   */
  public static function setMockedBaseFieldDefinitions(string $entity_type_id, array $definitions) {
    static::$mockedBaseFieldDefinitions[$entity_type_id] = $definitions;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
    $fields = parent::baseFieldDefinitions($entity_type);
    if (isset(static::$mockedBaseFieldDefinitions[$entity_type->id()])) {
      $mocked_fields = static::$mockedBaseFieldDefinitions[$entity_type->id()];
      // Mocked fields take priority over ones from the base class.
      $fields = $mocked_fields + $fields;
    }
    return $fields;
  }

}