The platforms I work on function solely on user defined input files. For unit tests we have example configuration files stored in the application as strings (and #ifdef'd away for non unit test builds). Then we split the file loading to allow loading from memory (string/stream).

This works so well, we actually package a library with our binary that contains the unit tests so the end user can run them locally without having to download/build anything. This has been super useful in identifying calculation differences between Linux/Windows/MacOS and processors. When a user reports an issue, we can ask them to run the unit tests just by running (./binary --test) and then give us the results.