Fix shipping_method basic test. Adapt to spree 2 that needs shipping_categories to be set and sets them on the base factory in a after_create block, the build command was not picking it up

spec/factories.rb

@@ -565,7 +565,7 @@ FactoryBot.modify do
     end
   end
 
-  factory :shipping_method do
+  factory :shipping_method, parent: :base_shipping_method do
     distributors { [Enterprise.is_distributor.first || FactoryBot.create(:distributor_enterprise)] }
     display_on ''
   end

spec/models/spree/shipping_method_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 module Spree
   describe ShippingMethod do
     it "is valid when built from factory" do
-      build(:shipping_method).should be_valid
+      create(:shipping_method).should be_valid
     end
 
     it "can have distributors" do