Set picking type on PO generated from sourcing with FA

This bridge with onchanges on purchase_delivery_address
and make it consistant with purchase_requisition_delivery_address
@nbessi indeed nesting helper methods helps to DRY, but sometimes I personally find it hard to follow and somewhat verbose (less repetition but more boilerplate).

Instead often I prefer to have shorter, more expressive test by splitting them in functionally significant parts (small realistic scenarios instead of pages-long stories), and avoiding data irrelevant to the test (sometimes with help from `Model.new()` and mocks). A bit of functionally relevant repetition in an expressive, short test is fine by me.

This is 100% subjective preference, of course: we all just want to make our tests readable and maintainable.