/**
 * Reads configuration data from the storage.
 * @return array|bool
 */
function readThing() {
  return ['module' => ['test' => 0]];
}

$extensions = readThing();
$extensions['theme']['test_basetheme'] = 0;
// This is the important part of the test. Themes are ordered alphabetically
// in core.extension so this will come before it's base theme.
$extensions['theme']['test_subtheme'] = 0;
$extensions['theme']['test_subsubtheme'] = 0;
unset($extensions['theme']['test_basetheme']);
unset($extensions['theme']['test_subsubtheme']);
unset($extensions['theme']['test_subtheme']);