A random thought on testing FreeType: I recently read about a small testing library for C code, here:
https://www.bassi.io/articles/2019/03/14/a-little-testing/. Might be interesting. Another option could be using the freetype-py wrapper and test with that (pytest is nice). One thing that would be welcome is a continuous integration ready approach, meaning that every commit would trigger test runs under different operating system images and compilers. This could help with uncovering differences due to compilers.