[api] Improve description of updating stock item test + add validation that count_on_hand is correct before proceeding
Create a product in stock location factory, rather than variants

This is so simply a master variant is created, rather than a master and another variant. The master variant still creates stock item records, so this factory will still behave as expected... it just won't create 4 variants when it only needs 2.
Remove commented out on_hand definition in variant factory