Add acceptance tests for external MDC events

We should add acceptance tests to make sure RAVEN and the related processes (GCN; trigger alert pipeline via 'RAVEN_ALERT') work before releasing a new version of gwcelery.

We should check that:

A coincidence was found (indicated by an 'EM_COINC' label on the external and superevent) for an applicable event
Check an alert was started by RAVEN indicated by RAVEN_ALERT, assuming publishing conditions are met
Check a emcoinc circular was created instead of a standard circular.

This should be in conjunction with #401 (closed) .

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