doc: clarify concurrency model of test runner

[tniessen]

This is based on my understanding of how it works, but I didn't actually test this or compare it with the implementation, so please review carefully @nodejs/test_runner.

Refs: #47365

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[MoLow]

LGTM, thanks

[GeoffreyBooth]

Maybe this adds clarity?

Suggested change

	then that many tests would run in parallel within the application thread.
	then that many tests would run in parallel within the current thread, similar to a set of promises awaited by `Promise.allSettled`.

[aduh95]

I don't think using current vs application does a great job at clarifying things. Why do you think it's important to call out Promise.allSettled vs anything else? I don't think it's helpful, it might even be a bit confusing given that the code doesn't use Promise.allSettled.

[GeoffreyBooth]

I’m trying to get across that it’s “in parallel” only in the sense that there are several async resources running within the current thread and process, as opposed to “true” parallelism from using multiple threads/processes.

[aduh95]

I think #47642 (review) is a better suggestion, I agree that "parallel" is simply not a great word to describe what happens here, but let's take that discussion to another PR/issue.

[benjamingr]

I would change the word "parallel" when it's used to "concurrently" when it applies to running in the same js thread and use "parallel" exclusively to refer to cases where multiple workers are used.

[nodejs-github-bot]

Landed in a51c894