Use require pkg to test core/x pkg

currently we use t for testing which is hard to write

I prefer not having require package in those core/x packages.
It’s better if they have no dependencies.

But in the case testing is really too limited and the tests are really complex, i don’t mind including require.

What is the downside of having external dependencies in core/x pkg?

To keep the packages as light as possible and independent from the decisions we make on the core’s specific packages.

I don’t see the point here, using another package to test a package shouldn’t be considered something complicated so, I don’t see a real life benefit about not using it here. Also x package itself already depending on lots of external dependencies and adding one more for testing is never an issue in my view.