tests: init more scalable unit tests

When trying this tool i discovered some bugs. I think we can fix them. But it would also require more extensive testing to ensure that functions that works right now also stay intact. **So this adds unit tests inspired by https://github.com/nix-community/rnix-parser** - Folders allow semantic grouping - Define a new test by just adding a new file. ```bash test_data/ └── write <- name of the action └── simple_insert <- semantic grouping, each unit_test consists of three files: ├── empty_attrs.args <- arguments: write("a", "1") ├── empty_attrs.expect <- { a = 1;} ├── empty_attrs.nix <- { } ├── existing_attrs.args ├── existing_attrs.expect └── existing_attrs.ni ```