Update tax_rate_factory

The amount is a decimal, so `0.1` here is actually a 10% tax rate. `100` is ridiculous (10000% tax).

Also, the factory fails without a calculator, and it should explicitly be a DefaultTax calculator or it will not be correct.

spec/factories/tax_rate_factory.rb

@@ -3,7 +3,8 @@
 FactoryBot.define do
   factory :tax_rate, class: Spree::TaxRate do
     zone
-    amount { 100.00 }
+    amount { 0.1 }
     tax_category
+    association(:calculator, factory: :default_tax_calculator, strategy: :build)
   end
 end