filechooserbutton: New test for opening the Other item in the combo box and then...

filechooserbutton: New test for opening the Other item in the combo box and then canceling the dialog

There is this bug:

1. Start with a file chooser button in SELECT_FOLDER mode, and select a folder from the combo box.

2. Click on the button's combo box, select 'Other...'

3. You get the file chooser dialog. Cancel the dialog.

4. The file chooser button's combo box still shows 'Other...' instead of
   showing the selection from (1).

This is a test to ensure that the original selection is restored.
The test fails right now.

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