drupal/core/modules/simpletest/simpletest.services.yml

services:
  test_discovery:
    # @todo Change this service so that it uses \Drupal\Core\Test\TestDiscovery
    # in https://www.drupal.org/node/1667822
    class: Drupal\simpletest\TestDiscovery
    arguments: ['@app.root', '@class_loader', '@module_handler']
  cache_context.test_discovery:
    class: Drupal\simpletest\Cache\Context\TestDiscoveryCacheContext
    arguments: ['@test_discovery', '@private_key']
    tags:
      - { name: cache.context}