[META] Improving maintainability and performance of JSON:API testsuite

Problem/Motivation

In the last few years JSON:API tests have been quite hard to debug and improve. They contain a lot of magic to test every single entity, also they test pretty much the full drupal stack including acces, caching and more.

This meta issue is mostly to work on how we are going improve things.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet