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) 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 or issue.

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 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.

先从 doc/api/cli.md 中关于 --experimental-test-isolation 的文档开始，然后阅读原始 PR #53927 和 issue #51548，了解现有的并发限制。跟踪 node --test 的入口点，并确定并发的非隔离 worker 应如何运行；完成标准包括支持所请求的 flag 组合并记录其语义。