Update scripts at root in package.json

[hegemonic]

I cloned this repo, then ran the following commands in its root directory:

npm i
npm run test

The command returns an error, because semistandard reports errors in multiple files.

[sofisl]

There are no tests at the head of this repo. I'll fix the command but to run tests you need to go into each directory in packages.

[hegemonic]

Yes, understood. But in #5724, I was making a change that affected the entire repo; I saw that there was a test target in the root package.json file, so I thought it was a good idea to run npm run test before sending a pull request.

If you don't want or need contributors to run npm run test in the repo root, please consider updating that target so it just prints a message telling users to run the command in the appropriate package directory.