Use the postgresql class to instantiate storage in tests

Test Plan

no more deprecation warnings in tox

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Migrated from D5881 (view on Phabricator)

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Patch application report for D5881 (id=21067)

Rebasing onto ad79654a...

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commit a21397ff7e49e617e418ce0bfa410991bd3a7056
Author: Nicolas Dandrimont <nicolas@dandrimont.eu>
Date:   Wed Jun 16 13:06:54 2021 +0200

    Use the postgresql class to instantiate storage in tests

See https://jenkins.softwareheritage.org/job/DLDBASE/job/tests-on-diff/482/ for more details.

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