Add csv export button do domain list in registrar area

#248

spec/requests/registrar/domains_controller_spec.rb

@@ -0,0 +1,28 @@
+require 'rails_helper'
+
+RSpec.describe Registrar::DomainsController, db: true do
+  describe '#index' do
+    before do
+      sign_in_to_registrar_area
+    end
+
+    it 'responds with success' do
+      csv_presenter = instance_double(Registrar::DomainListCSVPresenter, to_s: 'csv')
+      expect(Registrar::DomainListCSVPresenter).to receive(:new).and_return(csv_presenter)
+
+      get registrar_domains_url(format: 'csv')
+
+      expect(response.body).to eq('csv')
+    end
+
+    it 'returns csv' do
+      get registrar_domains_url(format: 'csv')
+
+      expect(response).to have_http_status(:success)
+    end
+  end
+
+  def sign_in_to_registrar_area(user: FactoryGirl.create(:api_user))
+    post registrar_sessions_path, { depp_user: { tag: user.username, password: user.password } }
+  end
+end