diff -u b/core/tests/bootstrap.php b/core/tests/bootstrap.php
--- b/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -11,6 +11,7 @@
 
 use Drupal\Component\Assertion\Handle;
 use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Site\Settings;
 use PHPUnit\Runner\Version;
 
 /**
@@ -32,8 +33,12 @@
 
   $extension_directories = [];
 
-  // Boot Drupal to be able to use the ExtensionDiscovery.
   $root = dirname(dirname(__DIR__));
+  $class_loader = require __DIR__ . '/../../autoload.php';
+  // Settings must be initialized otherwise
+  // Settings::get('file_scan_ignore_directories', []) returns an empty array
+  // in ExtensionDiscovery::scanDirectory().
+  Settings::initialize($root, 'sites/default', $class_loader);
   $discovery = new ExtensionDiscovery($root, FALSE);
 
   foreach (['module', 'profile', 'theme'] as $type) {