Run full RAVEN pipeline with test SNEWS events

We currently create external events with test SNEWS events but don't run the full RAVEN pipeline with these events (see: https://git.ligo.org/emfollow/gwcelery/-/blob/aebb33f48c7d691761519ce86e3957f25df5da89/gwcelery/tasks/external_triggers.py#L311-312)

We could test the full pipeline by instead including a condition in trigger_raven_alert to not publish with Test events (or MDC events for that matter).

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