Using SQLite database to store approved/received results

[nlohmann]

As follow up to #137, we see enormous performance issues as the number of tests that create output files grows over 100k files. Such a large number of files become unmanageable when working with Git.

I am thinking about writing the results to an SQLite database instead of individual files. I hope to achieve this with this framework and wonder what would be the best extension point to use for this. Is there a concept of using a different mechanism to store results already?