sql: move sql files in the python package and refactor tests to use these according to D538
This requires a few steps:
- refactor the BaseTestStorage to use SingleDbTestFixture
- rename any swh-xxx.sql files into NN-swh-xxx.sql so we can sort them without ambiguity
- move them into the python package, so the tests knows where to look for them.
Related to swh-storage#1265 (closed).
Test Plan
all tests should pass if swh-core!16 (closed) is applied to swh-core
Migrated from D543 (view on Phabricator)