test (unit/AP gateway): AP direct message ↔ XMPP `<message>` conversion: test that AP direct messages are converted to XMPP `<message>` stanzas instead of pubsub items. test that XMPP `<message>` stanzas are converted to AP direct messages (i.e. items addressed solely to the recipient, without *followers* or *public* collections). rel 366

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)