Issue #1881582 by sun: Change configuration overrides to use $config instead of $conf.

core/includes/bootstrap.inc

core/lib/Drupal/Core/Config/ConfigFactory.php

...les/config/lib/Drupal/config/Tests/ConfigOverrideTest.php

...g/lib/Drupal/config/Tests/ConfigOverridesPriorityTest.php

core/modules/simpletest/lib/Drupal/simpletest/TestBase.php

.../modules/simpletest/lib/Drupal/simpletest/WebTestBase.php

...b/Drupal/system/Tests/Installer/InstallerLanguageTest.php

...rupal/system/Tests/Installer/InstallerTranslationTest.php

.../modules/system/lib/Drupal/system/Tests/InstallerTest.php

core/modules/system/system.install

core/scripts/run-tests.sh

sites/default/default.settings.php