diff --git a/core/tests/Drupal/KernelTests/Setup/Commands/SetupDrupalTestScriptTest.php b/core/tests/Drupal/KernelTests/Setup/Commands/SetupDrupalTestScriptTest.php
index 668439cf17..c356efb94b 100644
--- a/core/tests/Drupal/KernelTests/Setup/Commands/SetupDrupalTestScriptTest.php
+++ b/core/tests/Drupal/KernelTests/Setup/Commands/SetupDrupalTestScriptTest.php
@@ -4,6 +4,8 @@
 
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\Setup\Commands\TestInstallationSetupApplication;
+use GuzzleHttp\Client;
+use GuzzleHttp\Psr7\Request;
 use Symfony\Component\Console\Tester\ApplicationTester;
 
 /**
@@ -17,20 +19,31 @@
  */
 class SetupDrupalTestScriptTest extends KernelTestBase {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    // Disable the usual kernel test setup process, as we want to run a custom
+    // command to install Drupal.
+    $this->root = static::getDrupalRoot();
+  }
+
   /**
    * @coversNothing
    */
   public function testInstallScript() {
-    $autoloader = dirname(dirname(dirname(__DIR__))) . '/bootstrap.php';
-    $app = new TestInstallationSetupApplication($autoloader);
+    $autoloader = $this->root . '/autoload.php';
+    $app = new TestInstallationSetupApplication(require $autoloader);
     $app->setAutoExit(FALSE);
 
     $app_tester = new ApplicationTester($app);
     $app_tester->run(
-      ['command' => 'setup-drupal-test'],
+      [
+        'command' => 'setup-drupal-test',
+        '--setup_file' => __DIR__ . '/SetupDrupalTestScript.php',
+      ],
       [
         'interactive' => FALSE,
-        'setup_file' => __DIR__ . '/SetupDrupalTestScript.php',
       ]
     );
 
@@ -38,6 +51,14 @@ public function testInstallScript() {
     $this->assertNotRegExp('/AlreadyInstalledException/', $app_tester->getDisplay());
     $this->assertRegExp('/simpletest/', $app_tester->getDisplay());
     $this->assertEqual(0, $app_tester->getStatusCode());
+
+    $http_client = new Client();
+    $request = (new Request('GET', getenv('SIMPLETEST_BASE_URL') . '/test-page'))
+      ->withHeader('User-Agent', trim($app_tester->getDisplay()));
+    
+    $response = $http_client->send($request);
+    // Ensure the test_page_test module got installed.
+    $this->assertContains('Test page | Drupal', (string) $response->getBody());
   }
 
 }
diff --git a/core/tests/Drupal/Setup/Commands/TestInstallationSetupApplication.php b/core/tests/Drupal/Setup/Commands/TestInstallationSetupApplication.php
index 94b42c6ec9..09761e00b0 100644
--- a/core/tests/Drupal/Setup/Commands/TestInstallationSetupApplication.php
+++ b/core/tests/Drupal/Setup/Commands/TestInstallationSetupApplication.php
@@ -17,8 +17,9 @@ class TestInstallationSetupApplication extends Application {
   /**
    * SetupDrupalApplication constructor.
    */
-  public function __construct() {
+  public function __construct($autoloader) {
     parent::__construct('setup-drupal-test', '0.0.1');
+    $this->setAutoloader($autoloader);
   }
 
   /**
diff --git a/core/tests/Drupal/Setup/TestInstallationSetup.php b/core/tests/Drupal/Setup/TestInstallationSetup.php
index 714b2815e4..f2fd50f0b2 100644
--- a/core/tests/Drupal/Setup/TestInstallationSetup.php
+++ b/core/tests/Drupal/Setup/TestInstallationSetup.php
@@ -91,23 +91,20 @@ protected function installDrupal() {
    *   The setup file.
    */
   protected function executeSetupFile($setup_file) {
-    $classes = static::fileGetPhpClasses($setup_file);
+    $class = static::fileGetPhpClass($setup_file);
 
-    if (empty($classes)) {
+    if (empty($class)) {
       throw new \InvalidArgumentException(sprintf('You need to define a class implementing \Drupal\Setup\TestSetupInterface'));
     }
-    if (count($classes) > 1) {
-      throw new \InvalidArgumentException(sprintf('You need to define a single class implementing \Drupal\Setup\TestSetupInterface'));
-    }
 
     require_once $setup_file;
 
-    if (!is_subclass_of($classes[0], TestSetupInterface::class)) {
+    if (!is_subclass_of($class, TestSetupInterface::class)) {
       throw new \InvalidArgumentException(sprintf('You need to define a class implementing \Drupal\Setup\TestSetupInterface'));
     }
 
     /** @var \Drupal\Setup\TestSetupInterface $instance */
-    $instance = new $classes[0];
+    $instance = new $class;
     $instance->setup();
   }
 
@@ -117,37 +114,52 @@ protected function executeSetupFile($setup_file) {
    * @param string $filepath
    *   The file path.
    *
-   * @return string[]
-   *   An array of PHP classes.
+   * @return string|null
+   *   Returns a PHP class.
    */
-  protected static function fileGetPhpClasses($filepath) {
+  protected static function fileGetPhpClass($filepath) {
     $php_code = file_get_contents($filepath);
-    $classes = static::extractClassesFromPhp($php_code);
-    return $classes;
+    return static::extractClassesFromPhp($php_code);
   }
 
   /**
    * @param string $php_code
    *   PHP code to parse.
    *
-   * @return string[]
-   *   An array of PHP classes.
+   * @return string
+   *   A PHP class.
    */
   protected static function extractClassesFromPhp($php_code) {
-    $classes = array();
     $tokens = token_get_all($php_code);
-    $count = count($tokens);
-    for ($i = 2; $i < $count; $i++) {
-      if ($tokens[$i - 2][0] == T_CLASS
-        && $tokens[$i - 1][0] == T_WHITESPACE
-        && $tokens[$i][0] == T_STRING
-      ) {
-
-        $class_name = $tokens[$i][1];
-        $classes[] = $class_name;
+
+    $class = $namespace = '';
+
+    for ($i = 0; $i < count($tokens); $i++) {
+      if ($tokens[$i][0] === T_NAMESPACE) {
+        for ($j = $i + 1; $j < count($tokens); $j++) {
+          if ($tokens[$j][0] === T_STRING) {
+            $namespace .= '\\' . $tokens[$j][1];
+          }
+          else {
+            if ($tokens[$j] === '{' || $tokens[$j] === ';') {
+              break;
+            }
+          }
+        }
       }
+
+      if ($tokens[$i][0] === T_CLASS) {
+        for ($j = $i + 1; $j < count($tokens); $j++) {
+          if ($tokens[$j] === '{') {
+            $class = $tokens[$i + 2][1];
+          }
+        }
+      }
+    }
+
+    if ($namespace && $class) {
+      return $namespace . '\\' . $class;
     }
-    return $classes;
   }
 
   /**