tests: init more scalable unit tests

[hsjobeki]

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.

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.nix