tests: refactor SingleDbTestFixture class to add generic db initialization

based on sql files shipped within the python package. This require several
small refactorings of SingleDbTestFixture.setUpClass and .add_db() methods:

• replace the 2 arguments (dump, dump_type) by a single 'dumps' one; this later must be an iterable of couples (dump_file, dump_type)
• refactor all needed helper functions accordingly.

We also add a small helper function to sort numerical file names, and provide a
DB_DUMP_TYPES as a global variable to prepare the next commit in which a new
cli tool 'swh-db-init' is added.

Related to T1265.