fix: stop a closed FakeServer from accepting one last connection

[9bany]

test_a_refused_connection_provably_never_arrived fails intermittently on CI with Unknown: [Errno 104] Connection reset by peer; the request was written in full and the outcome is not known - which is the opposite of what it asserts.

The test wants a port nobody is listening on, and builds one by starting a FakeServer and closing it. But close set a flag and shut the socket without joining the accept thread, and that thread is parked inside accept(). Closing a socket from another thread does not reliably wake a thread already blocked there, so the listener could outlive close by however long it took to notice. A client connecting in that window is accepted by a server that was supposed to be gone; with an empty script the handler reads the request in full and returns, dropping the connection. The client has written everything and cannot know whether it landed, which is exactly Unknown.

Fixed twice over, because the fixture bug is worth removing on its own:

• The test binds a socket, takes its port and closes it without ever calling listen. Nothing can be accepted on a socket that never listened, so the refusal is not a race.
• FakeServer now puts a timeout on the listening socket so the accept loop wakes to re-read its stop flag, and close joins the thread. After it returns, the server really has stopped - which is what every other test using it already assumed.

Not a client bug: Unknown was the correct thing to report for what actually happened on the wire.