Add link to openjdk.org/jtreg/ from doc/testing

doc/testing is a starting point for people interested in testing the JDK, but it doesn't explain jtreg and its nuances and pitfalls very well. https://openjdk.org/jtreg/, and expecially https://openjdk.org/jtreg/faq.html, contain better material. Let's at least add a pointer that will bring people there.