tests: Add test for PreviewWidget

The test fails currently because of the mainloop freezing, see issue #2278 (closed).

If you comment out this line in the test, it passes:

original_preview_uri(preview_widget, uri)

This is the PreviewWidget.preview_uri method, which calls _handle_new_asset which calls GES.UriClipAsset.request_sync(uri) which blocks!