diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
index 644de39..1f023ad 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
@@ -48,7 +48,7 @@ public function on500(GetResponseForExceptionEvent $event) {
 
     // When running inside the testing framework, we relay the errors
     // to the tested site by the way of HTTP headers.
-    if ($this->addHeaders()) {
+    if ($this->canAddHeaders()) {
       // $number does not use drupal_static as it should not be reset
       // as it uniquely identifies each PHP error.
       static $number = 0;
@@ -72,8 +72,8 @@ public function on500(GetResponseForExceptionEvent $event) {
    * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
    *   The event to process.
    */
-  public function onException(GetResponseEvent $event) {
-    if ($this->addHeaders()) {
+  public function onResponse(GetResponseEvent $event) {
+    if ($this->canAddHeaders()) {
       $response = $event->getResponse();
       $memory_usage = memory_get_usage();
       $response->headers->add([
@@ -98,8 +98,8 @@ public static function getSubscribedEvents() {
    * @return bool
    *   TRUE if headers should be added.
    */
-  protected function addHeaders() {
-    return (DRUPAL_TEST_IN_CHILD_SITE && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS)
+  protected function canAddHeaders() {
+    return (DRUPAL_TEST_IN_CHILD_SITE && !headers_sent() && (!defined('SIMPLETEST_COLLECT_ERRORS') || SIMPLETEST_COLLECT_ERRORS);
   }
 
 }
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 2146391..8733434 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1313,7 +1313,7 @@ protected function curlHeaderCallback($curlHandler, $header) {
     }
     if (!$this->ignoreMemoryUsage && preg_match('/^X-Drupal-Memory-Peak: (.*)$/', $header, $matches)) {
       if ((float) $matches[1] >= $this->memoryUsageTrigger) {
-        $this->fail(format_string('Child site memory usage (@siteMB) exceeded acceptable maximum (@maxMB) on url @url', [
+        $this->fail(SafeMarkup::format('Child site memory usage (@siteMB) exceeded acceptable maximum (@maxMB) on url @url', [
           '@url' => $this->getUrl(),
           '@site' => $matches[1],
           '@max' => $this->memoryUsageTrigger,