Add unit test for ImageListDesigner (#11725)

* Add unit test for ImageListDesigner

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/ImageListDesignerTests.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+namespace System.Windows.Forms.Design.Tests;
+
+public sealed class ImageListDesignerTests
+{
+    [Fact]
+    public void ActionLists_Get_ShouldContainExpectedItems()
+    {
+        using ImageListDesigner imageListDesigner = new();
+        using ImageList imageList = new();
+        imageListDesigner.Initialize(imageList);
+
+        imageListDesigner.ActionLists.Should().NotBeNull();
+        imageListDesigner.ActionLists.Cast<object>().Should().HaveCount(1);
+        imageListDesigner.ActionLists[0].Should().BeOfType<ImageListActionList>();
+    }
+}