Document why `node:test` exists

### Affected URL(s) https://nodejs.org/api/test.html ### Description of the problem # Background The core module `node:test` was proposed in https://github.com/nodejs/node/issues/40954 and shipped at `Experimental` stability in 18.0.0 via https://github.com/nodejs/node/pull/42325. Dozens of people expressed support for the idea via 👍 reactions and written comments. The sentiment of commenters toward the feature is overwhelmingly supportive. # Problem TL;DR the questions asked in https://github.com/nodejs/node/issues/40954#issuecomment-1037512376 seem unanswered, reducing the likelihood that `node:test` becomes adopted. It's unclear what problems the project aims to solve by including a test runner in core. ...or should I say—given the broad support shown by participants in https://github.com/nodejs/node/issues/40954 —that they're unclear to _me_. 😕 - If it's relevant, my background: I'm primarily a maintainer of Node.js applications, ~5 years of experience doing so, and am not a maintainer of any OSS. - I'm unsure how "should X go in core?" questions are usually settled, or if there are heuristics that exist in docs somewhere or in the heads of regular contributors. I went looking and found https://github.com/nodejs/node/blob/68fb0bf553e2af3e0b61733d29e1e9ba7f73d9b2/doc/contributing/pull-requests.md#accept-that-there-are-different-opinions-about-what-belongs-in-nodejs, which doesn't address the question so much as COC concerns at the periphery of the question. The couple “sufficient to put a thing in core” criteria it does identify aren’t met by a testing framework. @bcomnes had a comment here - https://github.com/nodejs/node/issues/40954#issuecomment-1037512376 - that resonated very much with me: It distilled and synthesized a lot of rough ideas that commenters had submitted up to that point. IMO his comment effectively highlights the _variety_ of goals that people might have for the module. It seems to me that the questions he raised remain unanswered. cc @nodej…