Testing Ecto.multi

I struggled with this a little bit, so I thought it would be good to go along with the ecto-multi.md

This can either be added to directly to the ecto-multi.md, or in another document and linked there. Thoughts on this part?

Start by reading ecto-multi.md and identify the testing guidance the issue author found difficult. Document how to test Ecto.Multi there or in a separate linked document; done means the testing explanation is clear and available from ecto-multi.md.

Written by the indexing model from the issue text.