Add testfx repo to README (#1459)

Author: Youssef1313

README.md

@@ -29,6 +29,7 @@ There are many projects that you can use and contribute to, some of which are li
 - [.NET (dotnet/core)](https://github.com/dotnet/core)
 - [.NET Aspire (dotnet/aspire)](https://github.com/dotnet/aspire)
 - [.NET MAUI (dotnet/maui)](https://github.com/dotnet/maui)
+- [.NET testing, includes Microsoft.Testing.Platform and MSTest (microsoft/testfx)](https://github.com/microsoft/testfx)
 - [ASP.NET Core (dotnet/aspnetcore)](https://github.com/dotnet/aspnetcore)
 - [C# (dotnet/csharplang)](https://github.com/dotnet/csharplang)
 - [Entity Framework Core (dotnet/efcore)](https://github.com/dotnet/efcore)
@@ -39,6 +40,7 @@ There are many projects that you can use and contribute to, some of which are li
 - [WPF (dotnet/wpf)](https://github.com/dotnet/wpf)
 
 ### .NET Docs
+
 - [.NET docs (dotnet/docs)](https://github.com/dotnet/docs)
 - [.NET Aspire docs (dotnet/docs-aspire)](https://github.com/dotnet/docs-aspire)
 - [.NET MAUI docs (dotnet/docs-maui)](https://github.com/dotnet/docs-maui)