[IMP] l10n_it_edi: Add a test for credit note XML export.

At the moment, there is no test that checks that normal credit notes
(i.e. of type TD04, not reverse-charge refunds) are correctly exported
in FatturaPA.

So we're adding one. We're making it a complicated credit note (it
contains some negative amounts) in order to improve the test coverage.

closes odoo/odoo#122930

Signed-off-by: Josse Colpaert <jco@odoo.com>