Testing tutorial, especially for testing concurrent code.

Links:

• concurrent unit
• tempus fugit lib with code on GitHub
• http://tutorials.jenkov.com/java-concurrency/thread-safety.html
• https://garygregory.wordpress.com/2011/09/09/multi-threaded-unit-testing/
• http://winterbe.com/posts/2015/04/30/java8-concurrency-tutorial-synchronized-locks-examples/
• http://blog.takipi.com/java-8-stampedlocks-vs-readwritelocks-and-synchronized/