Add feature specs for note polling

Ref. #4032, !2084

spec/features/issues/note_polling_spec.rb

+require 'spec_helper'
+
+feature 'Issue notes polling' do
+  let!(:project) { create(:project, :public) }
+  let!(:issue) { create(:issue, project: project) }
+
+  background do
+    visit namespace_project_issue_path(project.namespace, project, issue)
+  end
+
+  scenario 'Another user adds a comment to an issue', js: true do
+    note = create(:note_on_issue, noteable: issue,
+                  project_id: project.project_id,
+                  note: 'Looks good!')
+    sleep 16 # refresh interval in notes.js.coffee, 15 seconds
+    expect(page).to have_selector("#note_#{note.id}", text: 'Looks good!')
+  end
+end