tests: fix command-queue race in mock-ssh-server exec handling

tests/conftest.py

@@ -0,0 +1,28 @@
+import threading
+from queue import Queue
+
+import mockssh.server
+
+
+def _handler_run(self):
+    # Identical to mockssh.server.Handler.run except that the command
+    # queue is created atomically. Upstream checks `chanid not in
+    # command_queues` and then assigns a fresh Queue, while paramiko's
+    # transport thread may concurrently create the queue and put the
+    # command into it via check_channel_exec_request(); the assignment
+    # then replaces that queue, the command is lost, and handle_client
+    # blocks on Queue.get() forever -- the client never receives an
+    # exit status. mock-ssh-server is unmaintained, so it is patched
+    # here instead of upstream.
+    self.transport.start_server(server=self)
+    while True:
+        channel = self.transport.accept()
+        if channel is None:
+            break
+        self.command_queues.setdefault(channel.chanid, Queue())
+        thread = threading.Thread(target=self.handle_client, args=(channel,))
+        thread.daemon = True
+        thread.start()
+
+
+mockssh.server.Handler.run = _handler_run