Test REST import/export of 'flagging' content entity

[Sam152]

I tried using flag with default_content, and while an export that seemed to contain all of the information for a specific flag (user and target entity id), I couldn't get the flag to import:

exception 'Symfony\Component\Serializer\Exception\UnexpectedValueException' [error] with message 'Type http://drupal.org/rest/type/flagging/flag does not correspond to an entity on this site.' in app/core/modules/hal/src/Normalizer/ContentEntityNormalizer.php:251