diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index d6a2968..4edd715 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -1164,6 +1164,8 @@ private function restoreEnvironment() {
     $_SESSION = array();
     if (PHP_SAPI != 'cli' && session_status() == PHP_SESSION_ACTIVE) {
       session_destroy();
+      $params = session_get_cookie_params();
+      setcookie(session_name(), '', REQUEST_TIME - 3600, $params['path'], $params['domain'], $params['secure'], $params['httponly']);
     }
     session_name($this->originalSessionName);
 
@@ -1533,15 +1535,4 @@ public function copyConfig(StorageInterface $source_storage, StorageInterface $t
       $target_storage->write($name, $source_storage->read($name));
     }
   }
-
-  /**
-   * Return the session name in use on the child site.
-   *
-   * @return string
-   *   The name of the session cookie.
-   */
-  public function sessionName() {
-    return $this->originalSessionName;
-  }
-
 }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 9078a1c..9bf4c87 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -777,6 +777,16 @@ protected function drupalLogout() {
   }
 
   /**
+   * Return the session name in use on the child site.
+   *
+   * @return string
+   *   The name of the session cookie.
+   */
+  public function getSessionName() {
+    return $this->session_name;
+  }
+
+  /**
    * Sets up a Drupal site for running functional and integration tests.
    *
    * Installs Drupal with the installation profile specified in
@@ -807,6 +817,12 @@ protected function setUp() {
       'pass_raw' => $this->randomName(),
     ));
 
+    // The simpletest child site currently uses the same session name as the
+    // execution environment.
+    // @todo: Introduce a setting such that the session name can be customized
+    // for the child site.
+    $this->session_name = $this->originalSessionName;
+
     // Reset the static batch to remove Simpletest's batch operations.
     $batch = &batch_get();
     $batch = array();
@@ -1223,9 +1239,6 @@ protected function curlInitialize() {
       if (!$result) {
         throw new \UnexpectedValueException('One or more cURL options could not be set.');
       }
-
-      // By default, the child session name should be the same as the parent.
-      $this->session_name = $this->sessionName();
     }
     // We set the user agent header on each request so as to use the current
     // time and a new uniqid.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php b/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php
index d9d641b..cc753b6 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Session/SessionHttpsTest.php
@@ -40,12 +40,12 @@ public function setUp() {
 
   protected function testHttpsSession() {
     if ($this->request->isSecure()) {
-      $secure_session_name = $this->sessionName();
-      $insecure_session_name = substr($this->sessionName(), 1);
+      $secure_session_name = $this->getSessionName();
+      $insecure_session_name = substr($this->getSessionName(), 1);
     }
     else {
-      $secure_session_name = 'S' . $this->sessionName();
-      $insecure_session_name = $this->sessionName();
+      $secure_session_name = 'S' . $this->getSessionName();
+      $insecure_session_name = $this->getSessionName();
     }
 
     $user = $this->drupalCreateUser(array('access administration pages'));
@@ -124,8 +124,8 @@ protected function testMixedModeSslSession() {
       return;
     }
     else {
-      $secure_session_name = 'S' . $this->sessionName();
-      $insecure_session_name = $this->sessionName();
+      $secure_session_name = 'S' . $this->getSessionName();
+      $insecure_session_name = $this->getSessionName();
     }
 
     // Enable secure pages.
@@ -231,12 +231,12 @@ protected function testMixedModeSslSession() {
    */
   protected function testCsrfTokenWithMixedModeSsl() {
     if ($this->request->isSecure()) {
-      $secure_session_name = $this->sessionName();
-      $insecure_session_name = substr($this->sessionName(), 1);
+      $secure_session_name = $this->getSessionName();
+      $insecure_session_name = substr($this->getSessionName(), 1);
     }
     else {
-      $secure_session_name = 'S' . $this->sessionName();
-      $insecure_session_name = $this->sessionName();
+      $secure_session_name = 'S' . $this->getSessionName();
+      $insecure_session_name = $this->getSessionName();
     }
 
     // Enable mixed mode SSL.