[REFACTOR] sealed classes.

Author: Gonzalo Diaz

src/algorithm_exercises_csharp_test/hackerrank/interview_preparation_kit/linked_list/FindMergeNode.Test.cs

@@ -5,7 +5,7 @@ namespace algorithm_exercises_csharp_test.hackerrank.interview_preparation_kit.l
 [TestClass]
 public class FindMergeNodeTest
 {
-  class FindMergeNodeTestCase()
+  private sealed class FindMergeNodeTestCase()
   {
     public string title = "";
     public LinkedList<int>.Node llist1 = new(0);
@@ -81,7 +81,8 @@ public void testLinkedListCycle()
       Assert.AreEqual(
         test.expected,
         result,
-        String.Format(
+        string.Format(
+          System.Globalization.CultureInfo.InvariantCulture,
           "{0} findMergeNode({1}, {2}) => must be: {3}",
           test.title,
           test.llist1,

src/algorithm_exercises_csharp_test/hackerrank/interview_preparation_kit/linked_list/LinkedListCycle.Test.cs

@@ -5,7 +5,7 @@ namespace algorithm_exercises_csharp_test.hackerrank.interview_preparation_kit.l
 [TestClass]
 public class LinkedListCycleTest
 {
-  class LinkedListCycleTestCase
+  private sealed class LinkedListCycleTestCase
   {
     public string title = "";
     public LinkedList<string>.Node? llist;
@@ -65,6 +65,7 @@ public void testLinkedListCycle()
         test.expected,
         result,
         String.Format(
+          System.Globalization.CultureInfo.InvariantCulture,
           "{0} testLinkedListCycle({1}) => must be: {2}",
           test.title,
           test.llist,