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

[Aleksey Shipilev]

On Thu, 23 Sep 2021 09:28:29 GMT, Jie Fu <jiefu at openjdk.org> wrote:

>> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   More fixes
>
> doc/testing.md line 107:
> 
>> 105: (`JTREG_KEYWORDS=\!headful`), or both.
>> 106: 
>> 107: - `tier4`: This test group includes every other test not covered by previous tiers. It includes,
> 
> `other test` => `other tests`

No, I think "every other X" puts "X" in singular form: https://www.merriam-webster.com/dictionary/every%20other.

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