If you want to upgrade your apps to CodeIgniter4, and want to utilize the existing test code,
my new project might help you.

CodeIgniter 3 to 4 Upgrade Helper
https://github.com/kenjis/ci3-to-4-upgrade-helper

Sample upgraded test code:
https://github.com/kenjis/ci3-to-4-news/tree/main/tests/app

See How to Upgrade Test Code from CI3 to CI4.

Any opinions or suggestions are welcome.