Method assert_not_in_executor

test/coroutine/test_futex.cpp:28–32

  }

  static void assert_not_in_executor(Executor& executor) {
    if (executor.is_running_in()) {
      ::abort();
    }
  }

  ::babylon::AlwaysUseNewThreadExecutor& executor {
      ::babylon::AlwaysUseNewThreadExecutor::instance()};