"Make sure that the $test_class..." text appearing in whilst running Drupal\simpletest\Tests\KernelTestBaseTest

php code is not inside a php tag!

Run Drupal\simpletest\Tests\KernelTestBaseTest from the cli and you will see lots of

# Make sure that the $test_class variable is defined when this file is included.
if ($test_class) {
}

dumped to stdOut.