Support `concurrency` when `--experimental-test-isolation` is set to `'none'`

### What is the problem this feature will solve? It would be nice to be able to run multiple "workers" when using the `none` test isolation mode. Today, according to the docs, when `--experimental-test-isolation` is set to `'none'`, it implies 1 concurrency: https://github.com/nodejs/node/blob/f270462c09ddfd770291a7c8a2cd204b2c63d730/doc/api/cli.md#L2252-L2264 However, `mocha` another test runner that does not isolate tests, does accept a concurrency flag: https://mochajs.org/#parallel-tests ### What is the feature you are proposing to solve the problem? We're looking to move off of `jest` because its test module isolation is extremely slow. We love the idea of using the node-native test runner with isolation disabled, instead of adopting another third-party framework like `mocha`. However, we'd need to write some custom code (e.g., using [`parallel`](https://www.gnu.org/software/parallel/)) to spin up `n` concurrent, isolation-disabled tests to effectively utilize all the cores available on our CI machine. The docs and other recent comments all indicate that when `--experimental-test-isolation` is set to `'none'`, concurrency must be `1`. However, I couldn't find the reasoning in [the original PR](https://github.com/nodejs/node/pull/53927) or [issue](https://github.com/nodejs/node/issues/51548). There's probably a good reason for this but, as someone not intimately familiar with the implementation, it's not obvious to me why we wouldn't be able to run non-isolated tests concurrently, like in `mocha`. ### What alternatives have you considered? I could probably use a tool like [`parallel`](https://www.gnu.org/software/parallel/) to spin up multiple calls to `node --test`. However, this would require me to also write code to split up all the test files between the parallel runs, etc.