Add Apple Clang build script to AppVeyor

[Sebanisu]

AppVeyor was added with my Add Tests pull request.
#7

I'm not an expert on apple or clang. I might be able to get it to work via google. Anyone have more info on how to setup the CI for apple? If not, I'll eventually add it by googling and trial and error.

1. Install updated cmake 3.18+
2. Install updated clang 12+
3. Set updated clang as default compiler
4. Run Cmake
5. Build Tests
6. Run Tests in verbose mode. (Boost.ut gives extra info on tests passing or failing in verbose)