Remove unnecessary quibble loader registration

[aduth]

🛠 Summary of changes

Updates how we use quibble for JavaScript module substitution in tests to avoid explicitly passing loader option to Mocha.

This resolves a warning that's logged when running tests, and will likely help avoid a future incompatibility when Node.js removes the --loader flag:

(node:34551) ExperimentalWarning: `--experimental-loader` may be removed in the future; instead use `register()`:
--import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register("quibble", pathToFileURL("./"));'
(Use `node --trace-warnings ...` to show where the warning was created)

It's no longer necessary to pass the loader option in newer versions of Node.js, because Quibble ✨ automagically ✨ registers the hooks behind the scenes.

For versions larger or equal to v20.6.0, there is no need to specify a --loader, as registering the loader happens automatically once you use the API.

https://github.com/testdouble/quibble#es-modules-support

📜 Testing Plan

Verify build passes.

Verify yarn test passes, and doesn't log the warning shown above.