Add code coverage for RichTextBoxActionList

Winforms.lutconfig

@@ -0,0 +1,6 @@
+<LUTConfig Version="1.0">
+  <Repository />
+  <ParallelBuilds>true</ParallelBuilds>
+  <ParallelTestRuns>true</ParallelTestRuns>
+  <TestCaseTimeout>180000</TestCaseTimeout>
+</LUTConfig>

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

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+// 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 Moq;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public sealed class RichTextBoxActionListTests
+{
+    //private readonly Mock<EditorServiceContext> _editorServiceContextMock = new();
+
+    private readonly RichTextBoxDesigner _designer = new();
+
+    private readonly RichTextBox _richTextBox = new();
+
+    public RichTextBoxActionListTests()
+    {
+        _richTextBox.Lines = ["Line1", "Line2", "Line3"];
+        _designer.Initialize(_richTextBox);
+        //_editorServiceContextMock.Setup(editorServiceContext => EditorServiceContext.EditValue(_designerObject, _designerMock.Object.Component, It.IsAny<string>()));
+    }
+
+    [Fact]
+    public void RichTextBoxActionList_EditLines()
+    {
+        _richTextBox.Lines = ["Line4", "Line5", "Line6"];
+        var actionList = new RichTextBoxActionList(_designer);
+        actionList.EditLines();
+
+        actionList.GetSortedActionItems().Count.Should().Be(1);
+    }
+}