ci: add a runner for macos
This is running from a venv, created with the packages from the requirements
file in the `test-setup-future-work` topic being discussed separately. We can
basically reuse the existing `test_c` configuration as-is, but there was one
small issue- if the clone from the previous CI run is still present in `/tmp`,
the clone for the current run fails. It's not a problem for the current setup
because a new container is instantiated, so the easy fix is to simply delete the
clone first. (The Windows CI avoids this by not cloning in the first place.)
It looks like it takes ~80m to run the whole test suite on the current system.
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