diff -u b/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php b/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php
--- b/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php
+++ b/core/tests/Drupal/FunctionalTests/Update/UpdatePathTestBase.php
@@ -159,7 +159,7 @@
     // Boot up Drupal into a state where calling the database API is possible.
     // This is used to initialize the database system, so we can load the dump
     // files.
-    $autoloader = require_once __DIR__ . '/../../../../../autoload.php';
+    $autoloader = require $this->root . '/autoload.php';
     $kernel = TestRunnerKernel::createFromRequest($request, $autoloader);
     $kernel->loadLegacyIncludes();
 
diff -u b/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
--- b/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -258,6 +258,13 @@
   protected $metaRefreshCount = 0;
 
   /**
+   * The app root.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
    * The original container.
    *
    * Move this to \Drupal\Core\Test\FunctionalTestSetupTrait once TestBase no