Use Estonian reference number format instead of ISO 11649

test/fixtures/invoices.yml

@@ -8,6 +8,7 @@ DEFAULTS: &DEFAULTS
   buyer_name: Jane Doe
   vat_rate: 0.1
   total: 16.50
+  reference_no: 13
 
 valid:
   <<: *DEFAULTS

test/fixtures/registrars.yml

@@ -12,6 +12,7 @@ bestnames:
   language: en
   billing_email: billing@example.com
   website: bestnames.test
+  reference_no: 13
 
 goodnames:
   name: Good Names
@@ -22,6 +23,7 @@ goodnames:
   vat_no: DE123456789
   accounting_customer_code: goodnames
   language: en
+  reference_no: 26
 
 not_in_use:
   name: any
@@ -31,3 +33,4 @@ not_in_use:
   country_code: US
   accounting_customer_code: any
   language: en
+  reference_no: 39