Finalize integration of Testcontainers in all modules

[bobeal]

Spring provides some test support with embedded DBs (e.g. for neo4j) or with some mockked abstractions (e.g. for Kafka), but they are not so easy to integrate and prone to break.

As it is now validated, use the docker-compose integration provided by Testcontainers, that allows us to reuse our existing configs without duplication effort.

Finally, make sure the integration is consistent between all the modules (this could even be - at least partially - in the shared lib).