tst_QApplication: Make sure the native style is being used

The tst_QApplication::args() auto-test sets the style to windows style
but there is no way to reset the style back to the native one. This
makes tst_QApplication::focusChanged() fail on Mac in some cases,
since not all the styles respond the same way to tab vs. strong
focus changes.

Change-Id: I91e39c1dd0fad4d90f3a13ab50a5e9758922ac28
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>

tests/auto/widgets/kernel/qapplication/tst_qapplication.cpp

@@ -516,6 +516,8 @@ void tst_QApplication::args()
     QCOMPARE( argv_out, args_out );
 
     delete [] argv;
+    // Make sure we switch back to native style.
+    QApplicationPrivate::styleOverride = QString();
 }
 
 void tst_QApplication::appName()