kfilewidgettest: prevent the unit test from altering ~/.config/kdeglobals

When running kfilewidgettest, the saved view settings of the open-file dialog not loaded from kdeglobals, therefore it uses the default view settings, then when the dialog is "accpeted" during the unit test, those default setings are written to ~/.config/kdeglobals, which overrides the user settings.

To test (before applying this patch), set the view mode to "compact" in the open file dialog, run this unittest then check the dialog again, it'll be using the "details" view mode.