Is the FreeType2 test suite public? It looks like the project's tests/ directory only contains a script for downloading a single font [1]. They have a fuzz test repo with what appears to be a corpus of only a few hundred tests [2]. For critical infrastructure, like FreeType, I'd expect hundreds of thousands if not millions of tests, just like SQLite [3].
I don’t know if this applies to freetype’s testing, but when I worked on software that operated on fonts in the past I quickly concluded that piracy was the only practical way to test it. You need a very large corpus of real fonts, which would be both very expensive and not redistributable. This meant that I did not open source my tests.
[1] https://gitlab.freedesktop.org/freetype/freetype/
[2] https://github.com/freetype/freetype2-testing/
[3] https://www.sqlite.org/testing.html