objects: Add test for instance _save methods

7335af1ea fixes a bug with the instance save() and when it would call
the related _save methods for related objects.

This patch adds a test for it so that we are sure it does not regress.

Change-Id: Ie4b731b4b419725482b64b7aa1f19e7c5474c349
Merge "objects: Add test for instance _save methods"