Add unit tests for LinkUtilities (#12647)

related #10453

Proposed changes
Add unit tests for LinkUtilities to test its GetIELinkBehavior and EnsureLinkFonts methods.

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

@@ -0,0 +1,66 @@
+// 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 System.Drawing;
+using Microsoft.Win32;
+
+namespace System.Windows.Forms.Tests;
+
+public class LinkUtilitiesTests
+{
+    [WinFormsFact]
+    public void LinkUtilities_GetIELinkBehavior_ReturnsExpected()
+    {
+        // Read the registry value
+        RegistryKey? key = Registry.CurrentUser.OpenSubKey(LinkUtilities.IEMainRegPath);
+        string? registryValue = key?.GetValue("Anchor Underline") as string;
+
+        // Determine the expected behavior based on the registry value
+        LinkBehavior expectedBehavior = registryValue switch
+        {
+            "no" => LinkBehavior.NeverUnderline,
+            "hover" => LinkBehavior.HoverUnderline,
+            "always" => LinkBehavior.AlwaysUnderline,
+            _ => LinkBehavior.AlwaysUnderline
+        };
+
+        LinkUtilities.GetIELinkBehavior().Should().Be(expectedBehavior);
+    }
+
+    [WinFormsTheory]
+    [InlineData(LinkBehavior.AlwaysUnderline)]
+    [InlineData(LinkBehavior.HoverUnderline)]
+    [InlineData(LinkBehavior.NeverUnderline)]
+    [InlineData(LinkBehavior.SystemDefault)]
+    public void LinkUtilities_EnsureLinkFonts_CreatesExpectedFonts(LinkBehavior behavior)
+    {
+        using Font baseFont = new("Arial", 12);
+        Font? linkFont = null;
+        Font? hoverLinkFont = null;
+
+        Action act = () => LinkUtilities.EnsureLinkFonts(baseFont, behavior, ref linkFont, ref hoverLinkFont);
+
+        act.Should().NotThrow();
+        linkFont.Should().BeOfType<Font>();
+        hoverLinkFont.Should().BeOfType<Font>();
+    }
+
+    [WinFormsTheory]
+    [InlineData(LinkBehavior.AlwaysUnderline)]
+    [InlineData(LinkBehavior.HoverUnderline)]
+    [InlineData(LinkBehavior.NeverUnderline)]
+    public void LinkUtilities_EnsureLinkFonts_CreatesExpectedFonts_WithActive(LinkBehavior behavior)
+    {
+        using Font baseFont = new("Arial", 12);
+        Font? linkFont = null;
+        Font? hoverLinkFont = null;
+
+        Action act = () => LinkUtilities.EnsureLinkFonts(baseFont, behavior, ref linkFont, ref hoverLinkFont, isActive: true);
+
+        act.Should().NotThrow();
+        linkFont.Should().BeOfType<Font>();
+        hoverLinkFont.Should().BeOfType<Font>();
+    }
+}