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

[yaffs-website] / web / core / profiles / testing / testing.info.yml

name: Testing
type: profile
description: 'Minimal profile for running tests. Includes absolutely required modules only.'
version: VERSION
core: 8.x
hidden: true
install:
  # Enable page_cache and dynamic_page_cache in testing, to ensure that as many
  # tests as possible run with them enabled.
  - drupal:page_cache
  - dynamic_page_cache
# @todo: Remove this in https://www.drupal.org/node/2352949
themes:
  - classy