Completing the task of replacing all NULL to nullptr in all the files in tests folder
(also substituting some "0" used as nullptr with nullptr)
REVIEW: 114822
The idea behind this class is to relieve the developer from having to
call xcb_destroy_window once it is no longer needed. That is having a
RAII approach to windows.
In addition the class provides some simple method wrappers for the most
common use cases inside KWin:
* map
* unmap
* setGeometry - basically a moveResizeWindow
* ...