[#1110] Use TagsSubstitutionConcern as engine for mail template tags

spec/views/admin/mail_templates/edit.html.haml_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+
+describe 'admin/mail_templates/edit.html.haml', type: :view do
+  let(:procedure) { create(:procedure) }
+  let(:mail_template) { create(:mail_template, procedure: procedure) }
+  let(:all_tags) { mail_template.tags(reject_legacy: false) }
+
+  before do
+    allow(view).to receive(:admin_procedure_mail_template_path).and_return("/toto")
+    allow(view).to receive(:admin_procedure_mail_templates_path).and_return("/toto")
+
+    assign(:mail_template, mail_template)
+  end
+
+  subject { render }
+
+  context "Legacy champs are not listed in the page" do
+    it { expect(all_tags).to include(include({ libelle: 'numero_dossier', is_legacy: true })) }
+    it { is_expected.not_to include("numero_dossier") }
+  end
+
+  context "Non-legacy champs are listed in the page" do
+    it { expect(all_tags).to include(include({ libelle: 'numéro du dossier' })) }
+    it { is_expected.to include("numéro du dossier") }
+  end
+end