Add test case for MonthCalendarDesigner (#11692)

* Add test case for MonthCalendarDesigner

* Add enable nullability

---------

Co-authored-by: Simon Zhao (BEYONDSOFT CONSULTING INC) <[email protected]>

Author: SimonZhao888

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

@@ -0,0 +1,35 @@
+// 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 MonthCalendarDesignerTests
+{
+    [Fact]
+    public void AutoResizeHandles_WithDefaultMonthCalendar_ShouldBeTrue()
+    {
+        using MonthCalendarDesigner monthCalendarDesigner = new();
+        using MonthCalendar monthCalendar = new();
+        monthCalendarDesigner.Initialize(monthCalendar);
+
+        monthCalendarDesigner.AutoResizeHandles.Should().BeTrue();
+    }
+
+    [Fact]
+    public void SelectionRules_WithDefaultMonthCalendar_ShouldReturnExpectedValue()
+    {
+        using MonthCalendarDesigner monthCalendarDesigner = new();
+        using MonthCalendar monthCalendar = new();
+        monthCalendarDesigner.Initialize(monthCalendar);
+
+        SelectionRules selectionRules;
+        using (new NoAssertContext())
+        {
+            selectionRules = monthCalendarDesigner.SelectionRules;
+        }
+
+        selectionRules.Should().Be(SelectionRules.BottomSizeable | SelectionRules.RightSizeable | SelectionRules.Moveable | SelectionRules.Visible);
+    }
+}