[DO NOT MERGE] FFI unit tests

[Akvear]

WARNING: LLM generated - read at your own risk

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I have adjusted the migration documentation (removed/changed API) in ./docs/source/migration-guide.
• I added appropriate Fixes: annotations to PR description.

Obviously no need for review (I didn't even fully read this) - this is something for interested readers. I would recommend looking at the .md file and abi.rs. There are definitely some good concepts here and some terrible ones.