pytest_plugin: Change dbname to storage to avoid clash in tests
Other similar fixtures in other modules which use the same "tests" db already. Clash can then happen when table names exists in different modules (e.g. dbversion exists both in scheduler and storage dbs).
Technically, that renames the db used by tests from "tests" to "storage".
Related to swh/meta$770
Test Plan
tox
Migrated from D4014 (view on Phabricator)