RFR: 8293224 Add link to openjdk.org/jtreg/ from doc/testing

[Iris Clark]

On Thu, 1 Sep 2022 16:16:59 GMT, Ludvig Janiuk <duke at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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PR: https://git.openjdk.org/jdk/pull/10122