Don't use native dialogs for tst_QApplication::closeAllWindows()

The test shows an application modal QMessageBox, but assumes that doing
so will be non-blocking, which for macOS is not the case (yet). Instead
of making the dialog window-modal, which would potentially affect the
logic of the test, we disable native dialogs. This should be fine, as
the purpose of the test is to test the is_closing logic of
closeAllWindows, which lives on a layer above the native dialogs.

Change-Id: I6d627984a6ca452b876f34404b669fce41a00851
Reviewed-by: Doris Verria <doris.verria@qt.io>
(cherry picked from commit ad31b5f4c700fc7a66032fe14becc66b44c9d584)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

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

@@ -886,6 +886,7 @@ void tst_QApplication::closeAllWindows()
 {
     int argc = 0;
     QApplication app(argc, nullptr);
+    app.setAttribute(Qt::AA_DontUseNativeDialogs, true);
 
     // create some windows
     new QWidget;