tests: Normalise signature of argc/argv to match QApplication's

Fixes crash when passing "platform -xcb", because argc was being
passed by value, decremented in QApplication, then going out
of bounds in QTest

src/qtquick/TestHelpers_qtquick.cpp

@@ -45,7 +45,7 @@ private:
 };
 }
 
-Platform_qtquick::Platform_qtquick(int argc, char *argv[])
+Platform_qtquick::Platform_qtquick(int &argc, char **argv)
     : Platform_qt(argc, argv)
 {
     new QApplication(argc, argv);