Better test support

[ferdinand-beyer]

Init encourages writing tests that do not use the framework at all. You can just call the component start functions directly, passing in dependencies, and mocking them as needed.

However, for larger-scoped tests, this is inconvenient. Init makes it easy to refactor, adding and removing dependencies. But this means tests have to be updated accordingly.

Furthermore, one should test the system configuration, making sure that all dependencies are valid.

A more convenient way would be to start with the production config and:

• only select the components we want
• remove components that should not run in tests, e.g. databases and clients to external APIs
• add test doubles

This can be done by transforming the configuration, but it would be cool to have a better API to do that.