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