Replace mocha and c8 with native node test runner

[chris48s]

You've already replaced chai with node's native assert library. It should also be possible to replace mocha and c8 with

node --test --experimental-test-coverage

This should work even back to node 18.

Useful blog post: https://evertpot.com/bigint-money-2/

Some notes based on a quick spike:

• By default node expects to find tests in tests dir or file prefixed with test*. You might need to switch from the .spec naming convention and rename test-helpers.js

• You need to explicitly import describe, it, and any test hooks you are using e.g:

  import { describe, it, before, beforeEach, afterEach } from 'node:test';
  

• Remember to set V8R_CACHE_NAME env var

• It is probably worth double checking that tests using nock and mock-cwd don't cause any issue before going all in on this