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 [1] as a potential
refactoring)
[1] https://forge.softwareheritage.org/source/swh-loader-core/browse/master/swh/loader/tests/test_init.py$46-164
Closes T2494