libtree: use custom binary for tests to package for darwin too

`libtree` builds and works on macos (at least in part), but the tests do not. In matrix chat it was suggested to use `clang-unwrapped` with `--target=` to create some `.so` files which could be tested.

I am willing to implement this; I have not yet scheduled the work, though.

Pinging @prusnak @rardiol as maintainers.