swh.loader.tests: Use snapshot_get_all_branches in check_snapshot

This migrates to stop using old snapshot-get. This allows to use more Snapshot objects for checks!

This also fixes build [1]

• [1] https://jenkins.softwareheritage.org/job/DLDBASE/job/tests/1035/console

Test Plan

tox

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Migrated from D3744 (view on Phabricator)