CI: macOS: install and run tests using runner beta

Try to run out test suite on macOS using the new Gitlab CI Runner beta...

TODO

• get as far as pip install .
• brew is very slow, 15 mins maybe on macOS 11, report upstream?
  • pretty sloe on 12 as well, got it a bit faster with various env vars
• currently missing latex, no idea how to install that.
  • two options with brew:
    • brew install mactex-no-gui: very slow, CI job took 40 mins,
    • brew install basictex, faster
    • both are missing "latexmk"
  • how to install latexmk?
  • is there some alternative way to install tex on mac?
    • I see stuff on internetz about "brew cask" and "--cask" and whatnot: is all that ancient or still relevant?
    • probably, but not from command-line.
  • update docs/macos_installation.md: at least it doesn't cover latex and I doubt pip install tex is the way to install latex on a mac!
• brew install zbar is slow
  • 3 or 4 mins? not that bad, has a lot of deps.
• set this to run-manually rather than on every commit. Its about 30 min job, even with "basictex"
• maybe fedora should be same