Add unit tests for ContextMenuStripGroupCollection (#11952)

Related #10773
Add unit test ContextMenuStripGroupCollectionTests.cs for public properties and method of the ContextMenuStripGroupCollection.
Enable nullability in ContextMenuStripGroupCollection.

Author: Philip-Wang01

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

@@ -0,0 +1,55 @@
+// 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 class ContextMenuStripGroupCollectionTests
+{
+    [Fact]
+    public void Indexer_ShouldCreateNewGroupIfKeyDoesNotExist()
+    {
+        ContextMenuStripGroupCollection collection = new();
+
+        ContextMenuStripGroup group = collection["newKey"];
+
+        group.Should().NotBeNull();
+        group.Should().BeOfType<ContextMenuStripGroup>();
+        collection.ContainsKey("newKey").Should().BeTrue();
+    }
+
+    [Fact]
+    public void ContainsKey_ShouldReturnFalseIfKeyDoesNotExist()
+    {
+        ContextMenuStripGroupCollection collection = new();
+
+        bool containsKey = collection.ContainsKey("nonExistentKey");
+
+        containsKey.Should().BeFalse();
+    }
+
+    [Fact]
+    public void Indexer_ShouldReturnSameGroupForSameKey()
+    {
+        ContextMenuStripGroupCollection collection = new();
+
+        ContextMenuStripGroup group1 = collection["newKey"];
+        ContextMenuStripGroup group2 = collection["newKey"];
+
+        group1.Should().BeSameAs(group2);
+    }
+
+    [Fact]
+    public void Indexer_ShouldReturnDifferentGroupsForDifferentKeys()
+    {
+        ContextMenuStripGroupCollection collection = new();
+
+        ContextMenuStripGroup group1 = collection["key1"];
+        ContextMenuStripGroup group2 = collection["key2"];
+
+        group1.Should().NotBeSameAs(group2);
+        collection.ContainsKey("key1").Should().BeTrue();
+        collection.ContainsKey("key2").Should().BeTrue();
+    }
+}