static void Main(string[] args)
    {
        var list  = new TestPointLayer();
        var test1 = new TestPoint(list);
        var test2 = new TestPoint(list);

        Debug.WriteLine(list.Tests.Count);
        // 2
    }
 public TestPoint(TestPointLayer layer) : base(layer)
 {
     layer.Tests.Add(this);
 }