Use Estonian reference number format instead of ISO 11649

test/models/billing/reference_no_test.rb

@@ -0,0 +1,13 @@
+require 'test_helper'
+
+class ReferenceNoTest < ActiveSupport::TestCase
+  def test_returns_format_regexp
+    format = /\A\d{2,20}\z/
+    assert_equal format, Billing::ReferenceNo::REGEXP
+  end
+
+  def test_generated_reference_number_conforms_to_format
+    reference_no = Billing::ReferenceNo.generate
+    assert_match Billing::ReferenceNo::REGEXP, reference_no
+  end
+end