diff --git a/core/modules/ckeditor/src/Plugin/Editor/CKEditor.php b/core/modules/ckeditor/src/Plugin/Editor/CKEditor.php
index b13dec6..7b5cd5c 100644
--- a/core/modules/ckeditor/src/Plugin/Editor/CKEditor.php
+++ b/core/modules/ckeditor/src/Plugin/Editor/CKEditor.php
@@ -322,7 +322,7 @@ public function getLangcodes() {
       $files = scandir('core/assets/vendor/ckeditor/lang');
       foreach ($files as $file) {
         if ($file[0] !== '.' && fnmatch('*.js', $file)) {
-          $langcode = basename($file, 'js');
+          $langcode = basename($file, '.js');
           $langcodes[$langcode] = $langcode;
         }
       }
diff --git a/core/modules/simpletest/src/RandomGeneratorTrait.php b/core/modules/simpletest/src/RandomGeneratorTrait.php
index 51c6927..2a22de2 100644
--- a/core/modules/simpletest/src/RandomGeneratorTrait.php
+++ b/core/modules/simpletest/src/RandomGeneratorTrait.php
@@ -36,8 +36,18 @@
    *
    * @see \Drupal\Component\Utility\Random::string()
    */
-  protected function randomString($length = 8) {
-    return $this->getRandomGenerator()->string($length, TRUE, array($this, 'randomStringValidate'));
+  public function randomString($length = 8) {
+    if ($length < 3) {
+      return $this->getRandomGenerator()->string($length, TRUE, array($this, 'randomStringValidate'));
+    }
+
+    // To prevent the introduction of random test failures, ensure that the
+    // returned string contains a character that needs to be escaped in HTML by
+    // injecting an ampersand into it.
+    $replacement_pos = floor($length / 2);
+    // Remove 1 from the length to account for the ampersand character.
+    $string = $this->getRandomGenerator()->string($length - 1, TRUE, array($this, 'randomStringValidate'));
+    return substr_replace($string, '&', $replacement_pos, 0);
   }
 
   /**
@@ -51,7 +61,7 @@ protected function randomString($length = 8) {
    * @return bool
    *   TRUE if the random string is valid, FALSE if not.
    */
-  protected function randomStringValidate($string) {
+  public function randomStringValidate($string) {
     // Consecutive spaces causes issues for
     // \Drupal\simpletest\WebTestBase::assertLink().
     if (preg_match('/\s{2,}/', $string)) {
@@ -100,7 +110,7 @@ protected function randomMachineName($length = 8) {
    *
    * @see \Drupal\Component\Utility\Random::object()
    */
-  protected function randomObject($size = 4) {
+  public function randomObject($size = 4) {
     return $this->getRandomGenerator()->object($size);
   }
 
diff --git a/core/modules/system/tests/src/Kernel/Extension/ModuleHandlerTest.php b/core/modules/system/tests/src/Kernel/Extension/ModuleHandlerTest.php
index fc689d2..125a11b 100644
--- a/core/modules/system/tests/src/Kernel/Extension/ModuleHandlerTest.php
+++ b/core/modules/system/tests/src/Kernel/Extension/ModuleHandlerTest.php
@@ -37,8 +37,8 @@ protected function setUp() {
   /**
    * {@inheritdoc}
    */
-  public function containerBuild(ContainerBuilder $container) {
-    parent::containerBuild($container);
+  public function register(ContainerBuilder $container) {
+    parent::register($container);
     // Put a fake route bumper on the container to be called during uninstall.
     $container
       ->register('router.dumper', 'Drupal\Core\Routing\NullMatcherDumper');
diff --git a/core/tests/Drupal/Tests/KernelTestBase.php b/core/tests/Drupal/Tests/KernelTestBase.php
index 4408485..238c3d9 100644
--- a/core/tests/Drupal/Tests/KernelTestBase.php
+++ b/core/tests/Drupal/Tests/KernelTestBase.php
@@ -559,7 +559,13 @@ public function register(ContainerBuilder $container) {
     }
 
     // @todo Remove this BC layer.
-    $this->containerBuild($container);
+    $defined_class = new \ReflectionMethod($this, 'containerBuild');
+    $defined_class = $defined_class->getDeclaringClass()->name;
+
+    if (__CLASS__ !== $defined_class) {
+      $this->containerBuild($container);
+      trigger_error('Use ::register() instead', E_USER_DEPRECATED);
+    }
   }
 
   /**
@@ -570,7 +576,6 @@ public function register(ContainerBuilder $container) {
    * @see \Drupal\Tests\KernelTestBase::register()
    */
   public function containerBuild(ContainerBuilder $container) {
-    trigger_error('Use ::register() instead', E_USER_DEPRECATED);
   }
 
   /**