Add unit test for ListControlUnboundActionList (#12770)

Related #10773

Proposed changes
Add unit test ListControlUnboundActionListTests.cs for public properties and methods of the ListControlUnboundActionList.
Enable nullability in ListControlUnboundActionListTests.cs.

Author: John-Qiao

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

@@ -0,0 +1,51 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using System.ComponentModel;
+using System.ComponentModel.Design;
+using Moq;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public sealed class ListControlUnboundActionListTests : IDisposable
+{
+    private readonly ComponentDesigner _designer;
+    private readonly Mock<IComponent> _componentMock;
+    private readonly ListControlUnboundActionList _actionList;
+
+    public ListControlUnboundActionListTests()
+    {
+        _designer = new();
+        _componentMock = new();
+        _designer.Initialize(_componentMock.Object);
+        _actionList = new(_designer);
+    }
+
+    public void Dispose() => _designer.Dispose();
+
+    [Fact]
+    public void Constructor_ShouldInitializeDesigner() => _actionList.Should().NotBeNull();
+
+    [Fact]
+    public void GetSortedActionItems_ShouldReturnCorrectItems()
+    {
+        DesignerActionItemCollection items = _actionList.GetSortedActionItems();
+
+        items.Should().NotBeNull();
+        items.Count.Should().Be(1);
+        items[0].Should().BeOfType<DesignerActionMethodItem>();
+        DesignerActionMethodItem methodItem = (DesignerActionMethodItem)items[0];
+        methodItem.DisplayName.Should().Be(SR.ListControlUnboundActionListEditItemsDisplayName);
+        methodItem.Category.Should().Be(SR.ItemsCategoryName);
+        methodItem.Description.Should().Be(SR.ListControlUnboundActionListEditItemsDescription);
+    }
+
+    [Fact]
+    public void InvokeItemsDialog_ShouldThrowException()
+    {
+        Action action = _actionList.InvokeItemsDialog;
+        action.Should().Throw<Exception>();
+    }
+}