Loaders: standalone task tests

For each loader, run the loader outside of the scheduler/celery plumbing for test purposes:

• #2134 (closed): Implement uniform loading CLI (deployed, data scheduler migration in progress [1])

• Implement test protocol

  • swh/devel/swh-storage!912 (closed): select random origin (deployed)
  • run loader
  • report status (icinga?)

• Define scheduling mechanism for tests (icinga?)

• [1] $568

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Migrated from T2120 (view on Phabricator)