Fix for the showing of the spin viewer window in the GUI tests.
The show_tree() method of the main GUI window class was not calling the custom
self.spin_viewer.Show() method, as required to set up the observer objects required to keep the spin
viewer window updated. The value of status.show_gui was blocking this. Instead the show argument
of this Show() method is being set to status.show_gui to allow the method to always be executed.
