tst_qmdisubwindow: WindowCloseButtonHint is a standard window flag, too

Implemented that way in src/widgets/widgets/qmdisubwindow.cpp:2081.

Change-Id: I82fdf0c04f1655a130c5e6a6f1e23d325d546ab3
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Use a fake directory model instead of QDirModel in item view tests.

The tests then have a predictable, stable environment and do not depend
on file system operations.

Task-number: QTBUG-23697
Change-Id: Ibbd356f8bd7419ec4a3a88d2c0b5cd0830049790
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>