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

[Jie Fu]

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

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`

-------------

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