Added new book check_urls=free-programming-books-de.md (#4512)

* Added two new books check_urls=free-programming-books-de.md

* Removed source as it is not a Book

free-programming-books-de.md

@@ -65,6 +65,7 @@
 ### C++
 
 * [Die Boost C++ Bibliotheken](http://dieboostcppbibliotheken.de)
+* [Lean Testing für C++-Programmierer (2018)](https://www.assets.dpunkt.de/openbooks/Openbook_Lean_Testing.pdf) - Andreas Spillner, Ulrich Breymann (PDF)
 * [Programmieren in C++: Aufbau](http://www.highscore.de/cpp/aufbau/)
 * [Programmieren in C++: Einführung](http://www.highscore.de/cpp/einfuehrung/)