Fix asyncio.Server.wait_closed()

It was a no-op when used as recommended (after close()).

I had to debug one test (test__sock_sendfile_native_failure) --
the cleanup sequence for the test fixture was botched.

Hopefully that's not a portend of problems in user code --
this has never worked so people may well be doing this wrong. :-(

Lib/asyncio/base_events.py

@@ -377,7 +377,7 @@ async def serve_forever(self):
             self._serving_forever_fut = None
 
     async def wait_closed(self):
-        if self._sockets is None or self._waiters is None:
+        if self._waiters is None or self._active_count == 0:
             return
         waiter = self._loop.create_future()
         self._waiters.append(waiter)

Lib/test/test_asyncio/test_base_events.py

@@ -2052,11 +2052,11 @@ def prepare(self):
 
         def cleanup():
             server.close()
-            self.run_loop(server.wait_closed())
             sock.close()
             if proto.transport is not None:
                 proto.transport.close()
                 self.run_loop(proto.wait_closed())
+            self.run_loop(server.wait_closed())
 
         self.addCleanup(cleanup)