Add feature spec for system notes

spec/features/issuables/system_notes_spec.rb

+require 'spec_helper'
+
+describe 'issuable system notes', feature: true do
+  let(:issue)         { create(:issue, project: project, author: user) }
+  let(:merge_request) { create(:merge_request, :simple, source_project: project) }
+  let(:project)       { create(:project, :public) }
+  let(:user)          { create(:user) }
+
+  before do
+    project.add_user(user, :master)
+    login_as(user)
+  end
+
+  [:issue, :merge_request].each do |issuable_type|
+    context "when #{issuable_type}" do
+      before do
+        issuable = issuable_type == :issue ? issue : merge_request
+
+        visit(edit_polymorphic_path([project.namespace.becomes(Namespace), project, issuable]))
+      end
+
+      it 'adds system note "description changed"' do
+        fill_in("#{issuable_type}_description", with: 'hello world')
+        click_button('Save changes')
+
+        expect(page).to have_content("#{user.name} #{user.to_reference} changed the description")
+      end
+    end
+  end
+end