Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / system / tests / modules / entity_test_third_party / entity_test_third_party.info.yml

name: 'Entity test third-party settings module'
type: module
description: 'Provides third-party settings for test entity types.'
package: Testing
version: VERSION
core: 8.x
dependencies:
  - drupal:entity_test