JPA messageMappers and mailboxMapper should be tested

As described here (https://github.com/chibenwa/james-project/blob/v3.0_roadmap/v3.0_mailet_inventory.adoc), we want, to concider a back-end as production ready, to have unit tests at the mapper level.

We need to write such generic tests for JPA. It can easily be done by defining a MapperProvider and its associated contract implementation.

We may have to unactivate some tests (MOVE, MailboxAnnotations, ... )