Fix a hanging test on Python 3

I'm not entirely sure why the test is hanging, but this seems clear
enough:

- the test setup spawns a (non-daemon) background thread that runs
  forever, or until it is told to quit by receiving a TCP packet on a
  certain port
- the test teardown tries to tell the background thread to quit (which
  doesn't work) and waits for that to happen
- as a result the entire test run hangs forever

This commit adds a timeout as an extra safety net so that the test run
will complete even if the clean shutdown procedure fails for some
reason.

tests/checker/telnetserver.py

@@ -49,7 +49,8 @@ class TelnetServerTest (LinkCheckTest):
 
     def setUp (self):
         """Start a new Telnet server in a new thread."""
-        self.port = start_server(self.host, 0)
+        deadline = time.time() + 2 * TIMEOUT
+        self.port = start_server(self.host, 0, deadline=deadline)
         self.assertFalse(self.port is None)
 
     def tearDown(self):
@@ -60,7 +61,7 @@ class TelnetServerTest (LinkCheckTest):
             pass
 
 
-def start_server (host, port):
+def start_server (host, port, deadline):
     # Instantiate Telnet server class and listen to host:port
     clients = []
     def on_connect(client):
@@ -68,7 +69,7 @@ def start_server (host, port):
         client.send("Telnet test server\n")
     server = miniboa.TelnetServer(port=port, address=host, on_connect=on_connect)
     port = server.server_socket.getsockname()[1]
-    t = threading.Thread(None, serve_forever, args=(server, clients))
+    t = threading.Thread(None, serve_forever, args=(server, clients, deadline))
     t.start()
     # wait for server to start up
     tries = 0
@@ -91,9 +92,12 @@ def stop_server (host, port):
     client.write("stop\n")
 
 
-def serve_forever(server, clients):
+def serve_forever(server, clients, deadline):
     """Run poll loop for server."""
     while True:
+        if time.time() > deadline:
+            # It could be that the test is very slow, but that's unlikely.
+            raise AssertionError('deadlock detected, aborting')
         server.poll()
         for client in clients:
             if client.active and client.cmd_ready: