RFR: 8274083: Update testing docs to mention tiered testing [v5]

[David Holmes]

On Thu, 23 Sep 2021 12:53:23 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

Marked as reviewed by dholmes (Reviewer).

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