These are tests to validate source compatibility through various changes to FIDL libraries. They do not test binary compatibility.
fidl/ there are three libraries before, during and after reflecting the state of a FIDL library before a change has begun, during the transition and after the transition is complete.
The C++ tests consist of four executables that are built but not run since compile-time source compatibility is what is being validated.
cpp/before.ccagainst the before library
cpp/before.ccagainst the during library
cpp/after.ccagainst the during library
cpp/after.ccagainst the after library