improve test setup with factory bot

spec/factories/currencies.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: currencies
+#
+#  id         :bigint           not null, primary key
+#  name       :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+FactoryBot.define do
+  factory :currency do
+    name { "CAKE" }
+  end
+end