pytest_plugin: Make sample_data an object with attributes instead of dict

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 #2494 (closed)

Depends on !865 (closed)

Test Plan

tox

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Migrated from D3593 (view on Phabricator)