[17.0][FIX] account_reconcile_model_oca: Populate partner when writeoff reconcile model has partner mapping

[Peluko]

This should fix #781. partner_id was not populated when the dict for the writeoff lines are generated. Now I've added some tests.

[OCA-git-bot]

Hi @etobella,
some modules you are maintaining are being modified, check this out!

[OCA-git-bot]

Hi @etobella,
some modules you are maintaining are being modified, check this out!

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[pedrobaeza]

Please do a rebase over latest 17.0 branch and squah the commits.

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.