Update free-programming-books-langs.md (#7569)

Added book on .NET Unit Testing.

books/free-programming-books-langs.md

@@ -1568,6 +1568,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [NuGet In-House Succinctly](https://www.syncfusion.com/ebooks/nuget-in-house-succinctly) - José Roberto Olivas Mendoza
 * [Rider Succinctly](https://www.syncfusion.com/ebooks/rider-succinctly) - Dmitri Nesteruk
 * [Under the Hood of .NET Memory Management](https://assets.red-gate.com/community/books/under-the-hood-of-net-memory-management.pdf) - Chris Farrell, Nick Harrison (PDF)
+* [Unit Testing Succinctly](https://www.syncfusion.com/succinctly-free-ebooks/unittesting) - Marc Clifton
 
 
 ### NewSQL