Set oban crontab to false in tests

Fix for local falling tests:

** (RuntimeError) could not lookup Ecto repo Pleroma.Repo because it was not started or it does not exist

after some errors from Oban.Crontab.Scheduler.State

Last message: {:continue, :start}
State: %Oban.Crontab.Scheduler.State{circuit: :enabled, conf: %Oban.Config{circuit_backoff: 30000, crontab: [{%Oban.Crontab.Cron{days: [:*], hours: [0], minutes: [0], months: [:*], weekdays: [:*]}, Pleroma.Workers.Cron.ClearOauthTokenWorker, []}, {%Oban.Crontab.Cron{days: [:*], hours: [:*], minutes: [0], months: [:*], weekdays: [:*]}, Pleroma.Workers.Cron.StatsWorker, []}, {%Oban.Crontab.Cron{days: [:*], hours: [:*], minutes: [:*], months: [:*], weekdays: [:*]}, Pleroma.Workers.Cron.PurgeExpiredActivitiesWorker, []}, {%Oban.Crontab.Cron{days: [:*], hours: [0], minutes: [0], months: [:*], weekdays: [0]}, Pleroma.Workers.Cron.DigestEmailsWorker, []}], name: Oban, node: "master_91b02cba@127.0.0.1", poll_interval: 1000, prefix: "public", prune: :disabled, prune_interval: 60000, prune_limit: 5000, queues: [], repo: Pleroma.Repo, rescue_after: 60, rescue_interval: 60000, shutdown_grace_period: 15000, verbose: false}, name: nil, poll_interval: 60000, poll_ref: nil}