RFR: 8274083: Update testing docs to mention tiered testing [v3]

[Magnus Ihse Bursie]

On Thu, 23 Sep 2021 09:19:42 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Now that OpenJDK has more or less complete `tier{1,2,3,4}` definitions, let's mention them in `testing.md`. 
>> 
>> Current patch is my braindump, I am open for suggestions :)
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More fixes

I think this looks overall good. Just do something about that GHA link and I'm perfectly happy! :)

doc/testing.md line 95:

> 93: the tests in `tier1` are carefully selected and optimized to run fast, and to run in the
> 94: most stable manner. The test failures in `tier1` are usually followed up on quickly, either
> 95: with fixes, or adding relevant tests to problem list. [GitHub Actions workflows](../.github/workflows/),

I don't think the link to the `.github` directory makes sense here. If there should be a link at all, it should point to information on how GHA works and how to enable these. And frankly I'm not sure if any such comprehensive documentation exists. :-( Perhaps in the Skara wiki? Otherwise the link is just distracting.

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Changes requested by ihse (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5615