The good part about a language runtime is there's a massive corpus of tests that are in that language and agnostic to the implementation language.

For a JS engine that's Test 262: https://github.com/tc39/test262

For node that's its unit tests which are mostly JS: https://github.com/nodejs/node/tree/main/test

Node also runs the web platform tests too: https://web-platform-tests.org/

Bun has a similar large corpus of JS/TS tests: https://github.com/oven-sh/bun/tree/main/test

You're right about general purpose rewrites, but language runtimes are a lot easier.