Set cost in Shipment#add_shipping_method

Running into weird total issues after https://github.com/spree/spree/commit/a7f16ac85ee7b250341eeb131a518922d931dd3d
See failing spec in https://github.com/spree/spree-adyen/pull/3
Append lib_name to dummy databases

Avoid conflict between test databases for spree core and extensions for
example on the same host
Enable images to be created in the API within the scope of a product/variant