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

[yaffs-website] / web / core / modules / config / tests / config_override_test / config_override_test.info.yml

name: 'Configuration override test'
type: module
package: Testing
version: VERSION
core: 8.x

dependencies:
  - drupal:block
  - drupal:block_content