code: @Test public void shouldBeThreadSafeWhenMutated() throws Exception {
    final Group group=new Group("group");
    final List<Resource> resources=new ArrayList<Resource>();
    final Resource r1=Resource.create("/some.css",ResourceType.CSS);
    resources.add(r1);
    WroTestUtils.runConcurrently(new Callable<Void>(){
      public Void call() throws Exception {
        if (new Random().nextBoolean()) {
          group.setResources(resources);
        }
   else {
          group.addResource(r1);
          group.replace(r1,resources);
        }
        return null;
      }
    }
  );
  }
  