diff --git a/core/modules/simpletest/src/BrowserTestBase.php b/core/modules/simpletest/src/BrowserTestBase.php
index dc933ac..0131508 100644
--- a/core/modules/simpletest/src/BrowserTestBase.php
+++ b/core/modules/simpletest/src/BrowserTestBase.php
@@ -11,6 +11,7 @@
 use Behat\Mink\Element\Element;
 use Behat\Mink\Mink;
 use Behat\Mink\Session;
+use Drupal\Component\Serialization\Yaml;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Cache\Cache;
@@ -144,6 +145,15 @@
   protected $configImporter;
 
   /**
+   * Set to TRUE to strict check all configuration saved.
+   *
+   * @see \Drupal\Core\Config\Testing\ConfigSchemaChecker
+   *
+   * @var bool
+   */
+  protected $strictConfigSchema = TRUE;
+
+  /**
    * The profile to install as a basis for testing.
    *
    * @var string
@@ -859,9 +869,22 @@ public function installDrupal() {
       file_put_contents($directory . '/settings.php', "\n\$test_class = '" . get_class($this) . "';\n" . 'include DRUPAL_ROOT . \'/\' . $site_path . \'/settings.testing.php\';' . "\n", FILE_APPEND);
     }
     $settings_services_file = DRUPAL_ROOT . '/' . $this->originalSiteDirectory . '/testing.services.yml';
-    if (file_exists($settings_services_file)) {
-      // Copy the testing-specific service overrides in place.
-      copy($settings_services_file, $directory . '/services.yml');
+    if (!file_exists($settings_services_file)) {
+      // Otherwise, use the default services as a starting point for overrides.
+      $settings_services_file = DRUPAL_ROOT . '/sites/default/default.services.yml';
+    }
+    // Copy the testing-specific service overrides in place.
+    copy($settings_services_file, $directory . '/services.yml');
+    if ($this->strictConfigSchema) {
+      // Add a listener to validate configuration schema on save.
+      $content = file_get_contents($directory . '/services.yml');
+      $services = Yaml::decode($content);
+      $services['services']['simpletest.config_schema_checker'] = [
+        'class' => 'Drupal\Core\Config\Testing\ConfigSchemaChecker',
+        'arguments' => ['@config.typed'],
+        'tags' => [['name' => 'event_subscriber']]
+      ];
+      file_put_contents($directory . '/services.yml', Yaml::encode($services));
     }
 
     // Since Drupal is bootstrapped already, install_begin_request() will not
@@ -903,9 +926,13 @@ public function installDrupal() {
     file_prepare_directory($this->privateFilesDirectory, FILE_CREATE_DIRECTORY);
     file_prepare_directory($this->tempFilesDirectory, FILE_CREATE_DIRECTORY);
     $config->getEditable('system.file')
-      ->set('path.private', $this->privateFilesDirectory)
       ->set('path.temporary', $this->tempFilesDirectory)
       ->save();
+    $settings = [];
+    $settings['settings']['file_private_path'] = (object) [
+      'value' => $this->privateFilesDirectory,
+      'required' => TRUE,
+    ];
 
     // Manually configure the test mail collector implementation to prevent
     // tests from sending out emails and collect them in state instead.
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index d9aee35..1d78ce6 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -1148,7 +1148,7 @@ public function __get($name) {
           return Settings::get('file_public_path', \Drupal::service('site.path') . '/files');
 
         case 'private_files_directory':
-          return $this->container->get('config.factory')->get('system.file')->get('path.private');
+          return Settings::get('file_private_path');
 
         case 'temp_files_directory':
           return file_directory_temp();