Use brewtopia for testing

... this has several benefits:

• all the latest dependency versions (and same as on macOS homebrew)
  • most importantly: yaml-cpp, spdlog, fmt, armadillo
• can include Utopia installation in docker image
• much faster pipeline: 4 minutes instead of 9
• ... also due to much fewer artefacts to move between jobs

Caveat: Utopia version is not updated for each pipeline, but only upon merge to master. But this is not a deal breaker; if Utopia development speeds up, the master pipeline could even be triggered by changes made there.

Other changes:

• There is an additional dependency now, fmt, because brew's spdlog installs it separately, so it cannot be included from within spdlog any longer.
• Improved (faster) cmake setup