cephadm: fix invalid use of mock called_with method

Fixes `AttributeError: 'called_with' is not a valid assertion` errors
that appear when running the unit tests with python3.12.

Part of an effort to get ceph tox environments passing on Python 3.12.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

src/cephadm/tests/test_agent.py

@@ -668,7 +668,7 @@ def recv(self, len: Optional[int] = None):
         agent.mgr_listener.run()
 
         # verify payload was correctly extracted
-        assert _handle_json_payload.called_with(json.loads(payload))
+        _handle_json_payload.assert_called_with(json.loads(payload))
         FakeConn.send.assert_called_once_with(b'ACK')
 
         # second run, with bad json data received