add unit test for UserControlDocumentDesigner (#12710)

Related #10773

Proposed changes
Add unit test UserControlDocumentDesignerTests.cs for public properties and method of the UserControlDocumentDesigner.UserControlDocumentDesigner.
Enable nullability in UserControlDocumentDesignerTests.cs.

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

@@ -0,0 +1,79 @@
+// 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.Design;
+using System.ComponentModel.Design.Serialization;
+using System.Drawing.Design;
+using System.Windows.Forms.Design.Behavior;
+using System.Windows.Forms.Design.Tests.Mocks;
+using Moq;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class UserControlDocumentDesignerTests
+{
+    [WinFormsFact]
+    public void Constructor_ShouldSetAutoResizeHandlesToTrue()
+    {
+        using UserControlDocumentDesigner designer = new();
+        using UserControl control = new();
+
+        Mock<IDesignerHost> designerHost = new();
+        Mock<IComponentChangeService> componentChangeService = new(MockBehavior.Strict);
+        designerHost
+            .Setup(s => s.GetService(typeof(IComponentChangeService)))
+            .Returns(componentChangeService.Object);
+        Mock<ISelectionService> selectionService = new(MockBehavior.Strict);
+        designerHost
+            .Setup(s => s.GetService(typeof(ISelectionService)))
+            .Returns(selectionService.Object);
+        designerHost
+            .Setup(s => s.GetService(typeof(IDesignerHost)))
+            .Returns(designerHost.Object);
+
+        var mockSite = MockSite.CreateMockSiteWithDesignerHost(designerHost.Object);
+        mockSite
+            .Setup(s => s.GetService(typeof(IExtenderProviderService)))
+            .Returns(null!);
+        mockSite
+            .Setup(s => s.GetService(typeof(IUIService)))
+            .Returns(null!);
+        mockSite
+            .Setup(s => s.GetService(typeof(IOverlayService)))
+            .Returns(null!);
+        mockSite
+            .Setup(s => s.GetService(typeof(IMenuCommandService)))
+            .Returns(null!);
+        mockSite
+            .Setup(s => s.GetService(typeof(INameCreationService)))
+            .Returns(null!);
+        mockSite
+            .Setup(s => s.GetService(typeof(IPropertyValueUIService)))
+            .Returns(null!);
+        Mock<IEventHandlerService> eventHandlerService = new(MockBehavior.Strict);
+        mockSite
+            .Setup(s => s.GetService(typeof(IEventHandlerService)))
+            .Returns(eventHandlerService.Object);
+        Mock<IDictionaryService> dictionaryService = new(MockBehavior.Strict);
+        dictionaryService
+            .Setup(s => s.GetValue(It.IsAny<object>()))
+            .Returns(null!);
+        dictionaryService
+            .Setup(s => s.SetValue(It.IsAny<object>(), It.IsAny<object>()));
+        mockSite
+            .Setup(s => s.GetService(typeof(IDictionaryService)))
+            .Returns(dictionaryService.Object);
+        mockSite
+            .Setup(s => s.GetService(typeof(ComponentCache)))
+            .Returns(null!);
+        mockSite
+            .Setup(s => s.GetService(typeof(BehaviorService)))
+            .Returns(null!);
+        control.Site = mockSite.Object;
+
+        designer.Initialize(control);
+        designer.AutoResizeHandles.Should().BeTrue();
+    }
+}