diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index e1c15fe..25dc9aa 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -517,11 +517,7 @@ function find_conf_path($http_host, $script_name, $require_settings = TRUE) {
 function config_get_config_directory($type = CONFIG_ACTIVE_DIRECTORY) {
   global $config_directories;
 
-  if ($test_prefix = drupal_valid_test_ua()) {
-    // @see Drupal\simpletest\WebTestBase::setUp()
-    $path = conf_path() . '/files/simpletest/' . substr($test_prefix, 10) . '/config_' . $type;
-  }
-  elseif (!empty($config_directories[$type])) {
+  if (!empty($config_directories[$type])) {
     // Allow a configuration directory path to be outside of webroot.
     if (empty($config_directories[$type]['absolute'])) {
       $path = conf_path() . '/files/' . $config_directories[$type]['path'];
@@ -2596,8 +2592,8 @@ function drupal_valid_test_ua($new_prefix = NULL) {
     return $test_prefix;
   }
 
-  if (isset($_SERVER['HTTP_USER_AGENT']) && preg_match("/^(simpletest\d+);(.+);(.+);(.+)$/", $_SERVER['HTTP_USER_AGENT'], $matches)) {
-    list(, $prefix, $time, $salt, $hmac) = $matches;
+  if (isset($_SERVER['HTTP_USER_AGENT']) && preg_match("/^(simpletest\d+);(.+);(.+);(.+)([YN])$/", $_SERVER['HTTP_USER_AGENT'], $matches)) {
+    list(, $prefix, $time, $salt, $hmac, $config) = $matches;
     $check_string =  $prefix . ';' . $time . ';' . $salt;
     // We use the salt from settings.php to make the HMAC key, since
     // the database is not yet initialized and we can't access any Drupal variables.
@@ -2608,6 +2604,11 @@ function drupal_valid_test_ua($new_prefix = NULL) {
     // and the HMAC must match.
     if ($time_diff >= 0 && $time_diff <= 5 && $hmac == drupal_hmac_base64($check_string, $key)) {
       $test_prefix = $prefix;
+      if ($config === 'Y') {
+        foreach (array(CONFIG_ACTIVE_DIRECTORY, CONFIG_STAGING_DIRECTORY) as $type) {
+          $GLOBALS['config_directories'][$type] = array('path' => 'simpletest/' . substr($test_prefix, 10) . '/config_' . $type);
+        }
+      }
       return $test_prefix;
     }
   }
@@ -2619,7 +2620,7 @@ function drupal_valid_test_ua($new_prefix = NULL) {
 /**
  * Generates a user agent string with a HMAC and timestamp for simpletest.
  */
-function drupal_generate_test_ua($prefix) {
+function drupal_generate_test_ua($prefix, $config = 'Y') {
   global $drupal_hash_salt;
   static $key;
 
@@ -2632,7 +2633,7 @@ function drupal_generate_test_ua($prefix) {
   // Generate a moderately secure HMAC based on the database credentials.
   $salt = uniqid('', TRUE);
   $check_string = $prefix . ';' . time() . ';' . $salt;
-  return $check_string . ';' . drupal_hmac_base64($check_string, $key);
+  return $check_string . ';' . drupal_hmac_base64($check_string, $key) . $config;
 }
 
 /**