Add unit tests for FocusableLabel

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/FocusableLabelTests.cs

@@ -0,0 +1,47 @@
+// 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;
+
+public class FocusableLabelTests : IDisposable
+{
+    private readonly FocusableLabel _focusableLabel = new();
+
+    public void Dispose() => _focusableLabel.Dispose();
+
+    [Fact]
+    public void UnderlineWhenFocused_ShouldBeTrue() => _focusableLabel.UnderlineWhenFocused.Should().BeTrue();
+
+    [Fact]
+    public void UnderlineWhenFocused_SetToFalse_ShouldBeFalse()
+    {
+        using FocusableLabel label = new();
+
+        label.UnderlineWhenFocused = false;
+
+        label.UnderlineWhenFocused.Should().BeFalse();
+    }
+
+    [Fact]
+    public void CreateParams_ClassName_ShouldBeNull()
+    {
+        using SubFocusableLabel subLabel = new();
+
+        subLabel.CreateParams?.ClassName.Should().BeNull();
+    }
+
+    private class SubFocusableLabel : FocusableLabel
+    {
+        public new CreateParams? CreateParams => base.CreateParams;
+    }
+
+    [Fact]
+    public void TabStop_ShouldBeTrue()
+    {
+        using FocusableLabel label = new();
+
+        label.TabStop.Should().BeTrue();
+    }
+}