filechooserbutton: Test that the expected filename is shown

filechooserbutton: Test that the expected filename is shown

We do some gymnastics to pull the string out of the GtkButton or the GtkComboBox that is
being used in GtkFileChooserButton to show the current selection when the dialog
is inactive - namely, we look for the subwidget with the correct ATK role, and pull its
accessible name.

Currently the test fails; this is https://bugzilla.gnome.org/show_bug.cgi?id=691040#c18

Signed-off-by: Federico Mena Quintero <federico@gnome.org>