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

[yaffs-website] / web / core / modules / block_content / tests / modules / block_content_test / block_content_test.info.yml

name: "Custom Block module tests"
type: module
description: "Support module for custom block related testing."
package: Testing
version: VERSION
core: 8.x
dependencies:
  - drupal:block_content