diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 61905f2..8a4de0e 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -3499,12 +3499,13 @@ function drupal_check_memory_limit($required, $memory_limit = NULL) {
 function file_public_path() {
   $base_path = settings()->get('file_public_path', conf_path() . '/files');
   if ($test_prefix = drupal_valid_test_ua()) {
+    // Append the testing suffix unless already given.
     // @see Drupal\simpletest\WebTestBase::setUp()
-    return $base_path . '/simpletest/' . substr($test_prefix, 10);
-  }
-  else {
-    return $base_path;
+    if (strpos($base_path, '/simpletest/' . substr($test_prefix, 10)) === FALSE) {
+      return $base_path . '/simpletest/' . substr($test_prefix, 10);
+    }
   }
+  return $base_path;
 }
 
 /**
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 8a28d64..285e809 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -803,6 +803,7 @@ protected function setUp() {
         NestedArray::setValue($GLOBALS['conf'], array_merge(array($config_base), explode('.', $name)), $value);
       }
     }
+    $this->settingsSet('file_public_path', $this->public_files_directory);
     // Execute the non-interactive installer.
     require_once DRUPAL_ROOT . '/core/includes/install.core.inc';
     install_drupal($settings);