All the windows created automatically by the PassFailJFrame framework are registered using PassFailJFrame.addTestWindow in `windowList` for disposing of when the test completes.

A test developer also uses PassFailJFrame.addTestWindow to register additional windows which are created independently.

All these windows are part of the test UI, thus the window closing handler could be associated with all the windows. This is already done for the instruction frame and for any windows passed with `testUI` methods.

Instead, `addTestWindow` could set the handler, which will make window closing handler functionality localised.

Then all the test windows added by test developer using `addTestWindow` will benefit from the common handler.