tests: Fix 'called_once_with' used instead of 'assert_called_once_with'.

A MagicMock object was being asserted with 'called_once_with()'
instead of 'assert_called_once_with()', likely an accidental
mistake.

As a MagicMock always returns another instance of MagicMock on
anything not previoysly set up, the `assert` was always passing,
even though we updated the Window.connect_zoom() implementation
and that should have failed.

In Python 3.12 this type of typo is caught and throws an exception.

tests/interface/test_main.py

@@ -483,11 +483,10 @@ def test_Window_connect_zoom():
     w._zoom_out = mock.MagicMock()
     w._zoom_out.connect = mock.MagicMock()
     widget = mock.MagicMock()
-    widget.zoomIn = mock.MagicMock()
-    widget.zoomOut = mock.MagicMock()
+    widget.set_zoom = mock.MagicMock()
     w.connect_zoom(widget)
-    assert w._zoom_in.connect.called_once_with(widget.zoomIn)
-    assert w._zoom_out.connect.called_once_with(widget.zoomOut)
+    w._zoom_in.connect.assert_called_once_with(widget.set_zoom)
+    w._zoom_out.connect.assert_called_once_with(widget.set_zoom)
 
 
 def test_Window_current_tab():