Wait for ajax request to complete when testing milestont drang-n-drop feature

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/features/milestones/milestones_spec.rb

 require 'rails_helper'
 
 describe 'Milestone draggable', feature: true, js: true do
+  include WaitForAjax
+
   let(:milestone) { create(:milestone, project: project, title: 8.14) }
   let(:project)   { create(:empty_project, :public) }
   let(:user)      { create(:user) }
@@ -74,6 +76,8 @@ def create_and_drag_issue(params = {})
 
     visit namespace_project_milestone_path(project.namespace, project, milestone)
     issue.drag_to(issue_target)
+
+    wait_for_ajax
   end
 
   def create_and_drag_merge_request(params = {})
@@ -82,5 +86,7 @@ def create_and_drag_merge_request(params = {})
     visit namespace_project_milestone_path(project.namespace, project, milestone)
     page.find("a[href='#tab-merge-requests']").click
     merge_request.drag_to(merge_request_target)
+
+    wait_for_ajax
   end
 end