tests: refactor the StorageTestFixture class and move sql files in the package

so we can build the db setup sql file on the fly:

• make it inherit from SingleDbTestFixture, so we can
• move sql files in swh/storage/sql/
• get rid og the setUpClass static method to reuse the one from SingleDbTestFixture,
• apply the required modifications to tests based on this StorageTestFixture.

Warnings:

• this requires D538,
• this will break tests in swh-vault.

Also add README.md to the MANIFEST.in file so it does not require
a too recent setuptool.