Add unit tests for SplitterDesigner (dotnet#12359)

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

@@ -0,0 +1,17 @@
+// 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 SplitterDesignerTests
+{
+    [Fact]
+    public void Constructor_SetsAutoResizeHandlesToTrue()
+    {
+        using SplitterDesigner designer = new();
+
+        designer.AutoResizeHandles.Should().BeTrue();
+    }
+}