tests: repair --random_activity_priority argument
This stopped working when the signature of Message.init changed in 06aee0c2 (CMFActivity.ActivityTool: Use uid for identity check instead of oid., 2019-09-17)
This stopped working when the signature of Message.init changed in 06aee0c2 (CMFActivity.ActivityTool: Use uid for identity check instead of oid., 2019-09-17)