instead of gitmodules, can't we use composer? I think we do so already for the intl component (or emoji? I don't remember)

We could indeed use a package repository for composer. This is what I do for the sass-spec repo in https://github.com/scssphp/scssphp/ (and the setup could be simplified to avoid duplicating the commit reference btw not yet actually: composer/composer#12678).

I was thinking about the composer.json in this directory:
https://github.com/symfony/symfony/tree/8.1/src/Symfony/Component/Emoji/Resources/bin

Thanks for the hints, I followed and implemented the Emoji component approach

To me, the big difference between the way the Emoji component works and the way this PR works is that the Emoji composer-based task is about installing things needed for specific tasks updating committed data files that are part of the component. Running the testsuite does not require running those commands.

In the case of the JsonPath testsuite (or the Yaml testsuite), the testsuite itself is a dev dependency.
I would put that composer package repository in the normal composer.json to avoid extra work for contributors.

Proposed a new approach in the last commit taking into account your comment @stof. Updating the test suite would need to edit two files, but I think it's worth it given how easy it makes it to install the suite.

I'm not a big fan of the path guessing in the test case depending on if you use the monorepo or the standalone component, but I'm not sure there's a better solution here?