Refactor deterministic test data
As a follow-up to !267 (merged) where forums are not yet shared with contacts. We were discussing the following points:
- change the test data creator API to not use a given number of contacts/private chats, but rather allow to pick specific contacts/private chats from the
main
function (effectively "lifting" the DSL up to that level) - allow cross-references from defined forums to contacts those can be shared with
- support read and sent flags for private messages (and forum posts?) in the DSL