Add tor CI config for AppVeyor

At the Rome meeting, it was discussed (apparently, I wasn't there that time) to have an AppVeyor config for tor. As I understand it (and please feel free to correct this ticket!), the idea is to have multiple CI systems running (which is a thing we already do!). For example, currently, we have Jenkins and we also have TravisCI for personal (Github-based) forks (as per #22636 (moved)): Jenkins tests (essentially) (Debian package-based) builds on master and known (supported) tor versions, while Travis tests anything any developer pushes (albeit only for GCC/Clang on Linux, because everything else is unsupported/slow).

We should setup an AppVeyor config for testing tor on Windows. Ideally, it should match the testing behaviour of our Jenkins/Travis builds, so that we don't get spurious errors on one system or another.