tests: Add test content generation util function
Tests back content_get and content_get_metadata as a first step. Plan to use those for the new and upcoming content_get_range function.
To be rebased on the current work on storage.
Test Plan
tox
to test only those new tests though:
pytest -v -m property_based ./swh/storage/tests/test_storage.py --disable-warnings
Migrated from D646 (view on Phabricator)