Testing principles

[ndr-brt]

As evidenced here: #208 (comment)
Maybe it can be a good idea to draft a document where list the testing principles to follow in the codebase, similar to the architecture-principles.md.

Maybe we should start to list here some and discuss about.

• Every component containing logic (like services, handlers, controllers...) should be covered by at least one unit test.
• Do not use mocks for data objects, as they can be easily instantiated using their builders/constructors.
• Only public methods should be called by a test.
• Tests that have to do with external systems should be marked with the @IntegrationTest annotation.
• ...