Js package generator Add test runner to dev dependencies

[leogermani]

Changes proposed in this Pull Request:

• Fixes the package.json file created by jetpack generator

Jetpack product discussion

#21563 (comment)

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Generate a JS package and make sure test runner ends up under devDependencies in packages.json

[github-actions]

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[leogermani]

@anomiex I also wanted to remove yoast polyfills from composer.json but js-packages use the same skeleton file so we'll have to change things a bit more if we want that. (Do we really need composer.json there? Just for the changelogger?)

[anomiex]

We need a composer.json because all our tooling is based around using that file to identify that the project really is a project and using it to configure CI and stuff.

Ideally we'd let a file in tools/cli/skeletons/js-packages/composer.json override tools/cli/skeletons/common/composer.json, but right now it's the other way around. Or we could partially undo #21436 to instead have it delete composerJson['require-dev']['yoast/phpunit-polyfills'];