This is a simple rename.

I did this so that we could match [Node's convention][0]:

> If a directory named `test` is encountered, the test runner will
> search it recursively for all all `.js`, `.cjs`, and `.mjs` files. All
> of these files are treated as test files, and do not need to match the
> specific naming convention detailed below. This is to accommodate
> projects that place all of their tests in a single `test` directory.

Notice that `npm run test:unit` now just runs `node --test` with no
arguments.

[0]: https://nodejs.org/docs/latest-v20.x/api/test.html#running-tests-from-the-command-line