diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index ba07282..4c4d630 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -706,7 +706,7 @@ function simpletest_clean_temporary_directories($directory = NULL) {
 
   if (is_dir($simpletest_root)) {
     // If the $directory is not valid string or NULL then get the type of it for
-    // debugging purposes in the watchdog error message below.
+    // debugging purposes in the error message below.
     $path = is_string($directory) && !empty($directory) ? $directory : gettype($directory);
     // Do not recognize any, except expected directory pattern as wrong
     // directory being passed accidentally may cause catastrophic consequences.
diff --git a/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php b/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php
index 05235f4..c5931c6 100644
--- a/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php
+++ b/core/modules/simpletest/src/Tests/UiCleanTemporaryDirectoriesTest.php
@@ -31,15 +31,19 @@ public function testCleanTemporaryDirectories() {
     $simpletest = \Drupal::root() . '/sites/simpletest';
     $test_site_directory = basename($this->siteDirectory);
     $clean_environment = t('Clean environment');
-    $this->drupalGet('admin/config/development/testing');
 
-    $this->assertFieldByXPath('//input', $clean_environment, 'Displayed the "Clean environment" button.');
     $this->assertNull(simpletest_clean_temporary_directories('sites/simpletest/test_site_directory'), 'The sites/simpletest/test_site_directory folder does not exist.');
 
-    $this->recurse_copy_test_site_directory("$simpletest/$test_site_directory", "$simpletest/test_site_directory");
+    $this->recurseCopyTestSiteDirectory("$simpletest/$test_site_directory", "$simpletest/test_site_directory");
+    $original = scandir("$simpletest/$test_site_directory");
+    $copied = scandir("$simpletest/test_site_directory");
 
-    $this->assertTrue(is_dir("$simpletest/test_site_directory"), 'The test site directory is copied to the sites/simpletest/test_site_directory folder.');
+    $this->assertEqual($original, $copied, 'The original and copied test site directories exist in the sites/simpletest folder and have the same structure.');
+
+    $this->drupalGet('admin/config/development/testing');
 
+    $this->assertFieldByXPath('//input', $clean_environment, 'Displayed the "Clean environment" button.');
+sleep(30);
     $this->drupalPostForm(NULL, [], $clean_environment);
 
     $this->assertText(t('Removed @count temporary directory.', ['@count' => 1]), 'Displayed a message: "Removed 1 temporary directory.".');
@@ -49,22 +53,33 @@ public function testCleanTemporaryDirectories() {
 
   /**
    * Creates the exact copy of the test site temporary directory.
+   *
+   * The permissions on the copied directory structure are intentionally lowered
+   * in order to check if the chmod() works as as expected in the
+   * simpletest_clean_temporary_directories() function.
    */
-  protected function recurse_copy_test_site_directory($source, $destination) {
+  protected function recurseCopyTestSiteDirectory($source, $destination) {
     $directory = opendir($source);
-    mkdir($destination);
+    if (!mkdir($destination)) {
+      throw new \Exception("The $destination directory cannot be created.");
+    }
 
-    while(($file = readdir($directory)) !== FALSE) {
+    while (($file = readdir($directory)) !== FALSE) {
       if ($file != '.' && $file != '..') {
         if (is_dir("$source/$file")) {
-          $this->recurse_copy_test_site_directory("$source/$file", "$destination/$file");
+          $this->recurseCopyTestSiteDirectory("$source/$file", "$destination/$file");
         }
         else {
-          copy("$source/$file", "$destination/$file");
+          if (!copy("$source/$file", "$destination/$file") || !chmod("$destination/$file", 0444)) {
+            throw new \Exception("The $file cannot be copied to $destination with 0444 permissions.");
+          }
         }
       }
     }
     closedir($directory);
+    if (!chmod($destination, 0555)) {
+      throw new \Exception("The permissions 0555 cannot be set on the $destination directory.");
+    }
   }
 
 }
diff --git a/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module b/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module
index da2d82b..f52be67 100644
--- a/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module
+++ b/core/modules/simpletest/tests/modules/testing_page_test/testing_page_test.module
@@ -18,5 +18,8 @@ function testing_page_test_form_alter(&$form, $form_state, $form_id) {
  * Removes test site directory when the 'Clean environment' button is pressed.
  */
 function testing_page_test_submit() {
-  simpletest_clean_temporary_directories('sites/simpletest/test_site_directory');
+  $cleaned = simpletest_clean_temporary_directories('sites/simpletest/test_site_directory');
+  if ($cleaned !== TRUE) {
+    throw new \Exception("The sites/simpletest/test_site_directory cannot be removed because of unsufficient permissions.");
+  }
 }