add admin creation spec

spec/features/admin/admin_creation_spec.rb

@@ -0,0 +1,24 @@
+require 'spec_helper'
+
+feature 'As an administrateur', js: true do
+  let(:administration) { create(:administration) }
+  let(:admin_email) { 'new_admin@gouv.fr' }
+
+  before do
+    perform_enqueued_jobs do
+      administration.invite_admin(admin_email)
+    end
+  end
+
+  scenario 'I can register' do
+    confirmation_email = open_email(admin_email)
+    token_params = confirmation_email.body.match(/token=[^"]+/)
+
+    visit "admin/activate?#{token_params}"
+    fill_in :administrateur_password, with: 'démarches-simplifiées-pwd'
+
+    click_button 'Continuer'
+
+    expect(page).to have_content 'Mot de passe enregistré'
+  end
+end