pytest_plugin: Do not expose the validate proxy storage

Only the storage module needs it (because it's manipulating data as dict, which i'm migrating away from in the intent to remove that validate proxy storage altogether but it's taking some time...)

Note: I moved the swh_storage_backend_fixture and swh_storage so they are placed before the other fixtures which use it (so the reading order is more natural).

This should fix the debian jenkins build [1]

• [1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DLDBASE/job/gbp-buildpackage/153/console

^ the feedback loop for that is not good at all...

Test Plan

tox

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Migrated from D3528 (view on Phabricator)