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/qtcommon/Platform_qt.cpp

@@ -194,7 +194,7 @@ bool Platform_qt::isGammaray()
     return is;
 }
 
-Platform_qt::Platform_qt(int argc, char *argv[])
+Platform_qt::Platform_qt(int &argc, char **argv)
 {
     // This CTOR is called before we have a QApplication