Clarify usage for native tests

[sweemer]

It wasn't clear to me at first that there are both TypeScript and Rust tests for the native examples, so I've attempted to clarify it a bit in the README.

By the way, the workflow actions are running pnpm build-and-test instead of yarn run test as stated in the README. If pnpm build-and-test is the new preferred way to run the tests then I can update that part in the README too.