Put back tests on MacOS

[brockho]

We have disabled our MacOS tests on Jenkins because the slaves could not get activated again. We should therefore move towards having MacOS tests running on AppVeyor.

[ttusar]

We should look at these sites for help:

• https://www.scivision.dev/appveyor-ci-windows-linux/
• https://www.appveyor.com/docs/byoc/mac/
• https://github.com/cdcseacave/openMVS/blob/master/.appveyor.yml

[ttusar]

Since CircleCI also supports MacOS, we might want to try that first. Sample configurations are given here, but we should also look at reusable configurations to avoid code duplication.

[olafmersmann]

Minimal build tests are running on MacOS now using Github Workflows.

[olafmersmann]

Do we want more tests or can we close the issue?