Default order_factory line_items_count to 1

This took 40 seconds off of the "core" specs on my computer (3min30s -> 2min50s).  I didn't measure the other suites.
Given the speedup it seems like a reasonable thing to do for the cases where you only really need a single item.

Closes #5085

Conflicts:
	core/lib/spree/testing_support/factories/order_factory.rb