Introducing arethusaMocks

First steps for #287 

Introduces `arethusaMocks`, as specified in #287. More or less just a proof of concept, but already quite helpful - some spec code was refactored in the process.

Adds some arethusa core services as mock objects. They all hide behind a function (to retrieve a new copy all the time) and can take a customization object to override defaults properties.

I think we can build on this and improve it over time.