6 files declare their use of ConfigTest in Drupal 8

ConfigEntityListTest.php in core/modules/config/tests/src/Functional/ConfigEntityListTest.php

ConfigInstallWebTest.php in core/modules/config/tests/src/Functional/ConfigInstallWebTest.php

ConfigTestController.php in core/modules/config/tests/config_test/src/ConfigTestController.php

ConfigTestResourceTestBase.php in core/modules/config/tests/config_test/tests/src/Functional/Rest/ConfigTestResourceTestBase.php

ConfigTestTest.php in core/modules/jsonapi/tests/src/Functional/ConfigTestTest.php

config_test.hooks.inc in core/modules/config/tests/config_test/config_test.hooks.inc

Fake third-party hook implementations for ConfigTest entities.