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;
    }

}