These are tests to validate source compatibility through various changes to FIDL libraries. They do not test binary compatibility.
Under 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 compiles cpp/before.cc against the before librarycpp_during_1 compiles cpp/before.cc against the during librarycpp_during_2 compiles cpp/after.cc against the during librarycpp_after compiles cpp/after.cc against the after library