RFR: Initial section on testing [v9]

[Iris Clark]

On Thu, 22 Oct 2020 20:11:27 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> It's a start.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Added note about TEST.properties

Marked as reviewed by iris (Reviewer).

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PR: https://git.openjdk.java.net/guide/pull/30