Make Elasticsearch unit tests run against an Elasticsearch cluster in PostgreSQL/MongoDB environments

Since NXP-28618, the unit tests of the nuxeo-elasticsearch modules should be running against an Elasticsearch cluster (not embedded) in the PostgreSQL and MongoDB environments.

We've realized, thanks to NXP-29529, that it wasn't the case.
Indeed, the related PR was showing all checks having passed, while still relying on the Elasticsearch 6 Helm chart. In such a context, the utests/postgresql and utests/mongodb statuses should have failed, as the code migrated to Elasticsearch 7 cannot run against Elasticsearch 6.