Write your own unit test framework

UnitTest.cs

using System;
using System.Linq;
using System.Collections.Generic;

using JerryUnit;
using static JerryUnit.UnitTestAttribute;
          
public class Program
{
  public static void Main()
  {
    TestRunner.Run(new Custom.MyLibraryTests());
    Console.WriteLine($"{Environment.NewLine}End");
  }
}

namespace Custom
{
  public static class MyLibrary
  {
    public static string Reverse(string text)
    {
      if (text is null)
        throw new ArgumentNullException(nameof(text));
      var array = text.ToCharArray();
      Array.Reverse(array);
      return new String(array);
    }
  }

  public class MyLibraryTests
  {
    [UnitTest]
    public void Reverse_String_Reversed() =>
		Reverse("Jerry", "yrreJ");

    [UnitTest]
    public void Reverse_Empty_Empty() =>
		Reverse(string.Empty, string.Empty);

    [UnitTest]
    public void Reverse_Null_ArgumentNullException()
    {
      try { MyLibrary.Reverse(null); }
      catch (ArgumentNullException) { return; }
      Assert(false, "Expected ArgumentNullException");
    }

    private static void Reverse(string original, string expected) 
	{
		var actual = MyLibrary.Reverse(original);
		Assert(expected == actual, $"Expected '{expected}' == '{actual}'");
	}
  }  
}

namespace JerryUnit
{
  public class UnitTestAttribute : Attribute 
  {
    public static void Assert(bool result, string message = null)
    {
      if (!result) throw new Exception(message);
    }
  }

  public static class TestRunner
  {
    public static void Run(object list)
    {
      if (list is null)
        throw new ArgumentNullException(nameof(list));

      var tests = FindTests(list);
      if (!tests.Any())
        throw new System.ArgumentException("Test list is empty.");

      RunTests(list, tests).ToList().ForEach(x => Console.WriteLine(((x.Success) 
        ? $"✅ Pass: {(list)}.{x.Name}" 
        : $"❌ Fail: {(list)}.{x.Name} > Error: {x.Message}")));

      static IEnumerable<System.Reflection.MethodInfo> FindTests(object list) =>
        list.GetType().GetMethods().Where(x => x.GetCustomAttributes(typeof(UnitTestAttribute), false).Any());

      static IEnumerable<(bool Success, string Name, string Message)> RunTests(object list, IEnumerable<System.Reflection.MethodInfo> tests) 
      {
        foreach (var test in tests)
        {
          var message = string.Empty;
          try { test.Invoke(list, null); }
          catch (Exception e) { message = (e.InnerException is not null) ? e.InnerException.Message : e.Message; }
          yield return (string.IsNullOrEmpty(message), test.Name, message);
        }
      }
    }
  }
}

https://dotnetfiddle.net/ZYVV1K