The X11 specific tests need to run on QT_QPA_PLATFORM xcb otherwise they will crash. To enforce this without having to replace QTEST_MAIN a Q_CONSTRUCTOR_FUNCTION is used to invoke a function which does nothing except setting the env variable.
It was not doing anything in addition and the documentation was completely wrong as it does the same thing as QTEST_MAIN.
Follows the naming schemes in frameworks and opens up the possibility to include test applications for KWin in the tests subdirectory.
