message: More stringent.
diff: @@ -5,6 +5,8 @@ output_control: ob_* in coroutine
  --FILE--
  <?php
  require_once __DIR__ . '/../../include/bootstrap.php';
  +ob_start();
  +echo 'main';
  // #co1
  go(function () {
  ob_start();
  @@ -40,6 +42,7 @@ go(function () {
  co::sleep(0.2);
  assert(ob_get_status(true) === []); //empty
  });
  +assert(ob_get_clean() === 'main');
  ?>
  --EXPECT--
  foo
  