Server.wait_closed() currently does two checks:

1. if _sockets is None -- means that Server.close() was called
2. if self._waiters is None -- means that Server._wakeup() was called

if (1) *or* (2) is true, wait_closed() just returns without waiting on anything.

However, when Server.close() is called there might be still active transports serving requests.  Server.wait_closed() should wait until all of them are detached, even if Server._sockets is already reset.

So the below implementation:

    async def wait_closed(self):
        if self._sockets is None or self._waiters is None:
            return
        waiter = self._loop.create_future()
        self._waiters.append(waiter)
        await waiter

should be changed to:

    async def wait_closed(self):
        if self._waiters is None:
            assert self._active_count == 0
            return
        waiter = self._loop.create_future()
        self._waiters.append(waiter)
        await waiter