[12.0] backports from 13.0

[nilshamerlinck]

Backports of

• [13.0][IMP] queue_job: check that queue_job_notify trigger exists in db #264
• [13.0][ADD] queue_job: innocuous test job for debugging/monitoring purposes #265
• [13.0][FIX] run doctests as part of standard tests #296

[dreispt]

@nilshamerlinck Could you be kind to resolve the conflicts please? I'll then move on with the merge. Thanks!

[nilshamerlinck]

Hi @dreispt actually I will close it, as it's been superseded by the more ambitious #333 :)