Restructure test folder with same layout of source files

[leoll2]

Refactor the tests/unit/experimental folder to match the layout of the corresponding source files. Likewise, align the names of the tests with the names of the tested functions/classes. This organization ensures that unit tests are easy to find and clearly linked with the respective code.

Example: method LabelIndexConverter.convert defined in experimental/converters/annotation_converters.py should be tested by TestLabelIndexConverter::test_convert in tests/unit/experimental/converters/test_annotation_converters.py