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

[yaffs-website] / web / core / modules / system / tests / modules / test_page_test / test_page_test.links.menu.yml

test_page_test.test_page:
  route_name: test_page_test.test_page
  title: 'Test front page link'
  weight: 0
test_page_test.front_page:
  title: 'Home'
  route_name: '<front>'
  menu_name: main