[skip-ci] Fix stressGUI and Makefile.win32

Discovered when running `stressGUI`:
- Fix a couple of PATHs in `stressGUI.cxx`
- Fix the compilation of `Aclock`, `Hello`, and `Tetris` and add support for `Win64` in `test/Makefile.win32`
- Fix the following error with `tutorials/gui/Slider3Demo.C`:
  ```
  tutorials\gui\Slider3Demo.C:198:21: error: call to member function 'SetPosition' is ambiguous
  ```