Add milestone deprecation feature spec

spec/features/milestone_spec.rb

@@ -108,4 +108,18 @@ feature 'Milestone' do
       expect(page).to have_selector('.js-delete-milestone-button', count: 0)
     end
   end
+
+  feature 'deprecation popover', :js do
+    it 'opens deprecation popover' do
+      milestone = create(:milestone, project: project)
+
+      visit group_milestone_path(group, milestone, title: milestone.title)
+
+      expect(page).to have_selector('.milestone-deprecation-message')
+
+      find('.milestone-deprecation-message .popover-link').click
+
+      expect(page).to have_selector('.milestone-deprecation-message .popover')
+    end
+  end
 end