Fix doc of DbTestFixture.
Summary: DbTestFixture.setupClass reads cls._DB_DUMP_LIST, which is set by cls.add_db, so cls.add_db should be called by the child's setupClass beforehand
Reviewers: Reviewers, ardumont
Reviewed By: Reviewers, ardumont
Subscribers: ardumont, swh-public-ci
Differential Revision: https://forge.softwareheritage.org/D603