community/chez-scheme: move from testing

Has been in testing/ for almost two years and works quite well in my experience. Furthermore, the package is now available for all architectures supported by upstream and tests are enabled and pass. As such, I don't see any reason to keep it in testing/ any longer and it is also needed as a dependency of other packages (e.g. idris2) and hence would be neat to have in community/ to also be able to move these other packages to community/ in the future.