RFR: JDK-8296240: Augment discussion of test tiers in doc/testing.md [v6]

[Alan Bateman]

On Tue, 31 Oct 2023 16:11:47 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> Clarify the intention of tier 1 tests. I'll reflow the paragraph and regenerate the HTML file once the wording is agreed upon.
>
> Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update wording.

Marked as reviewed by alanb (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16384#pullrequestreview-1711155231