winrt: Fix tst_QDialog::showFullScreen

show will use the platform's default flags but the test actually checks
for the flags that were set before so we have to use setVisible(true)
instead.

Change-Id: I52c055ec07d6f0dee626626318c875aefdf67484
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp

@@ -339,7 +339,7 @@ void tst_QDialog::showFullScreen()
     QVERIFY(dialog.isFullScreen());
     QVERIFY(!dialog.isVisible());
 
-    dialog.show();
+    dialog.setVisible(true);
     QVERIFY(dialog.isFullScreen());
     QVERIFY(dialog.isVisible());