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

[yaffs-website] / web / core / modules / filter / tests / filter_test / filter_test.info.yml

name: 'Filter test module'
type: module
description: 'Tests filter hooks and functions.'
package: Testing
version: VERSION
core: 8.x
dependencies:
  - drupal:filter