Fix test_root_window_model by formatting geometry string correctly

You can't use `{geometry:24}` style formatting on a list or tuple. It
must be converted to a string first.

xpra/server/shadow/root_window_model.py

@@ -65,7 +65,7 @@ def __init__(self, root_window, capture=None, title="", geometry=None):
         self.signal_listeners = {}
 
     def __repr__(self):
-        return f"RootWindowModel({self.capture} : {self.geometry:24})"
+        return f"RootWindowModel({self.capture} : {str(self.geometry):24})"
 
     def get_info(self) -> dict:
         info = {}