Use the shipping calculators in the test factories +

The shipping calculators are similar but not the same as the calculators
used for other models; the shipping calculators are contained in the
Spree::Calculators::Shipping namespace. The shipment_method_factory
doesn't make the distinction between the two calculators.

Support the currency property on the flat rate calculator (fixes #5433)+

This enables promotions to have differing values for different
currencies, or for a promotion to only apply for a single currency.

The flat rate calculator's currency and the order's currency are
compared, if they don't match the calculator returns 0, and 0-rate
promos don't get added to orders.

Fixes #5698