diff --git a/core/modules/simpletest/src/Tests/KernelTestBaseTest.php b/core/modules/simpletest/src/Tests/KernelTestBaseTest.php
index b37c74d..218d0b9 100644
--- a/core/modules/simpletest/src/Tests/KernelTestBaseTest.php
+++ b/core/modules/simpletest/src/Tests/KernelTestBaseTest.php
@@ -27,7 +27,10 @@ class KernelTestBaseTest extends KernelTestBase {
    * {@inheritdoc}
    */
   protected function setUp() {
-    $settings_file = <<<'EOS'
+    $php = <<<'EOS'
+# Make sure that the $test_class variable is defined when this file is included.
+if ($test_class) {
+}
 <?php
 # Define a function to be able to check that this file was loaded with
 # function_exists().
@@ -37,7 +40,7 @@ function simpletest_test_stub_settings_function() {}
 EOS;
 
     $settings_testing_file = $this->siteDirectory . '/settings.testing.php';
-    file_put_contents($settings_testing_file, $settings_file);
+    file_put_contents($settings_testing_file, $php);
 
     $original_container = $this->originalContainer;
     parent::setUp();