Note that this:
- drops the no longer needed copy done by the StorageData instance (used by sample_data) since now it returned immutable BaseModel objects.
- centralizes some left-over tests to use sample_data as well
This concludes the tests refactoring about model objects. Now all objects used
within tests are immutable BaseModel objects.
Furthermore, the storage exposes the sample_data fixture so it can be reused by
client parts without having to redefine their own (see  as a potential
Depends on D3592