These auto tests were trying to not create native dialogs, but setting
the QFileDialog::DontUseNativeDialog option is too late to control the
creation of the platform helper. It is already created at construction
time, unless Qt::AA_DontUseNativeDialogs is set.
Task-number: QTBUG-55276
Task-number: QTBUG-55281
Change-Id: Icf474e97059ac03a5fa01bd3a17f07203da5770a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>