Fix qmltests::WebEngineViewSingleFileUpload auto tests

Add scheme checks for FilePickerController::accepted().
A file:// prefix to a file selected here causes chromium to terminate
the render process due to an illegal ipc message.

Extend auto tests with custom Dialog.

Also fix directory upload test.

Change-Id: I8d672e435129d62d268b48c3471b924c0161b44b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>