Refactor the swh_scheduler fixture

2 points:

• make it more generic so that stuff like the indexer_scheduler fixture in swh-indexer!89 (closed) is not needed any more
• better handling of the call to run_ready_tasks() as pointed by @vlorentz on https://forge.softwareheritage.org/!44

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Migrated from T1473 (view on Phabricator)