RFR: Initial section on testing [v6]

[Joe Darcy]

On Tue, 20 Oct 2020 10:50:28 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:
> 
>   Link to html version of testing docs

I think the addition of some conceptual testing information is sufficient for now, with the possibility of adding more
discussion later.

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Marked as reviewed by darcy (no project role).

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