This provide several advantages:
- more flexibility in designing unit tests
- unit tests can be glued to speed up compilation
- unit tests are compiled with same predefined macros, which is a requirement for zapcc
- fix usage of Index (API) versus StorageIndex (when multiple indexes are stored)
- use StorageIndex(val) when the input has already been check
- use internal::convert_index<StorageIndex>(val) when val is potentially unsafe (directly comes from user input)
