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)