[FIX] l10n_it_edi_sdicoop: if to the PA, it should be changed to sent

First thing is that when you send an invoice to PA in test/production,
it should be considered as sent after the xml is generated as we do not
have the functionality to send sign invoices yet.

We added a test for posting the invoice when the invoice partner has a
l10n_it_pa_index (and it is of length 6).

Alos, when the proxy user is a 'demo' user, creates a
traceback. The solution is to only add the demo response to those that
would have been sent to the iap server otherwise.

We also avoid doing a call to the IAP server if there are no invoices to be
sent (e.g. because they all go to the PA)

opw-2807213

closes odoo/odoo#91020

Signed-off-by: William André (wan) <wan@odoo.com>