improve test setup with factory bot

spec/factories/fiat_balances.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: fiat_balances
+#
+#  id              :bigint           not null, primary key
+#  amount_cents    :integer          default(0), not null
+#  amount_currency :string           default("BRL"), not null
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#  user_id         :bigint           not null
+#
+# Indexes
+#
+#  index_fiat_balances_on_user_id  (user_id)
+#
+# Foreign Keys
+#
+#  fk_rails_...  (user_id => users.id)
+#
+FactoryBot.define do
+  factory :fiat_balance do
+    association :user
+    amount_currency { "BRL" }
+    amount_cents { Faker::Number.number(digits: 10) }
+  end
+end