Convert specs to tests

#623

test/models/invoice_test.rb

@@ -8,4 +8,16 @@ class InvoiceTest < ActiveSupport::TestCase
   def test_valid
     assert @invoice.valid?
   end
+
+  def test_calculates_subtotal
+    assert_equal BigDecimal('15'), @invoice.sum_without_vat
+  end
+
+  def test_calculates_vat_amount
+    assert_equal BigDecimal('1.5'), @invoice.vat
+  end
+
+  def test_calculates_total
+    assert_equal BigDecimal('16.5'), @invoice.sum
+  end
 end