🛠 Repo: Add tests for issues discovered with native import() requiring

[JoshuaKGoldberg]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Mocha 11.7.0 included #5366, using require to load ESM when supported in the running Node.js version. This was a great feature add for fixing #5314 & #5317, as well as working towards unblocking nodejs/node#57298.

However, it uncovered at least 2-3 user-reported issues:

• 🐛 Bug: Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './esm-register' is not defined by "exports #5380
• feat: use require to load esm #5366 (comment): asynchronous Node.js loaders/hooks
• feat: use require to load esm #5366 (comment): MDN's browser-compat data tests

The root issue here is that the Mocha repo doesn't test those cases. Which is a problem. We should really have test coverage for these core user flows IMO.

Additional Info

IMO a good next step would be to:

1. Fill in any examples in https://github.com/mochajs/mocha-examples
2. Set up "ecosystem" style tests for PRs to make sure all examples are fixed

I think that can be separately tracked / out of scope for this specific issue.