I don't think it's that easy for the typical unit test though?
Most of the time usage need to advance time in controlled fashion. So you'll need to create a shell wrapper that runs faketime with correct FAKETIME_TIMESTAMP_FILE value, hook it up to your build system, create C++ wrapper to write to this file... If you are controlling source code, adding a "time" argument might be easier to implement and easier to understand.
libfaketime's awkward interface (control via timestamp files, needing LD_PRELOAD) is why I wrote an experimental library[0] which takes over the clock at the process level.
My library modifies the code for the time-related vDSO functions, which is incredibly sketchy yet effective. There are also Python bindings at [1].
Most of the time usage need to advance time in controlled fashion. So you'll need to create a shell wrapper that runs faketime with correct FAKETIME_TIMESTAMP_FILE value, hook it up to your build system, create C++ wrapper to write to this file... If you are controlling source code, adding a "time" argument might be easier to implement and easier to understand.