RFR: Initial section on testing

[Phil Race]

On Fri, 16 Oct 2020 15:58:52 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

> It's a start.

src/index.md line 378:

> 376:
> 377: The [JTReg documentation](https://openjdk.java.net/jtreg/) provides information on many more tags like these.
> 378:

Also jtreg supports the @key tag which may have a value used to filter test selection. As a specific example if you
have a UI test which needs to display a window, you should specify @key headful
This one is important to make sure the test harness does not try to run tests on a system which does not support those
tests.  Make sure you understand the conventions for the particular area you are testing since this is just an example.

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