Add unit tests for CommandSet (dotnet#12769)

Related dotnet#10773

Proposed changes
Add unit test CommandSetTests.cs for public method of the CommandSet.
Enable nullability in CommandSetTests.

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

@@ -0,0 +1,45 @@
+// 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 Moq;
+using System.ComponentModel;
+using System.ComponentModel.Design;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class CommandSetTests
+{
+    [Fact]
+    public void Dispose_DisposesResourcesCorrectly()
+    {
+        Mock<ISite> mockSite = new();
+        Mock<IEventHandlerService> mockEventHandlerService = new();
+        Mock<IDesignerHost> mockDesignerHost = new();
+        Mock<IMenuCommandService> mockMenuCommandService = new();
+        Mock<ISelectionService> mockSelectionService = new();
+        Mock<IDictionaryService> mockDictionaryService = new();
+
+        mockSite.Setup(s => s.GetService(typeof(IEventHandlerService))).Returns(mockEventHandlerService.Object);
+        mockSite.Setup(s => s.GetService(typeof(IDesignerHost))).Returns(mockDesignerHost.Object);
+        mockSite.Setup(s => s.GetService(typeof(IMenuCommandService))).Returns(mockMenuCommandService.Object);
+        mockSite.Setup(s => s.GetService(typeof(ISelectionService))).Returns(mockSelectionService.Object);
+        mockSite.Setup(s => s.GetService(typeof(IDictionaryService))).Returns(mockDictionaryService.Object);
+
+        dynamic accessor;
+        using (CommandSet commandSet = new(mockSite.Object))
+        {
+            accessor = commandSet.TestAccessor().Dynamic;
+        }
+
+        mockMenuCommandService.Verify(m => m.RemoveCommand(It.IsAny<MenuCommand>()), Times.AtLeastOnce);
+        mockEventHandlerService.VerifyRemove(e => e.EventHandlerChanged -= It.IsAny<EventHandler>(), Times.Once);
+        mockDesignerHost.VerifyRemove(h => h.Activated -= It.IsAny<EventHandler>(), Times.Once);
+        mockSelectionService.VerifyRemove(s => s.SelectionChanged -= It.IsAny<EventHandler>(), Times.Once);
+
+        ((object)accessor.SelectionService).Should().BeNull();
+        ((object)accessor.BehaviorService).Should().BeNull();
+        ((object)accessor._menuService).Should().BeNull();
+    }
+}