Add some model tests (#9)

Reviewed-on: #9

test/models/credit_card_bill_test.rb

@@ -1,7 +1,9 @@
 require "test_helper"
 
 class CreditCardBillTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  def test_unpaid
+    credit_card_bill = credit_card_bills(:one)
+
+    assert_equal 870.00, credit_card_bill.unpaid
+  end
 end