Add a macOS worker to the CI

This is built on #549 but
I've made in a separate PR because I'm not sure if there will be other issues
here and I don't want to block that PR on this (but at the same time, tests
won't pass on Mac without the changes from that PR).

See #513 and #507

.github/workflows/tests.yaml

@@ -15,8 +15,7 @@ jobs:
     strategy:
       matrix:
         # cannot run on windows due to needing fake-chroot for conda-docker
-        # osx takes forever to get a scheduled job
-        os: [ubuntu-latest]
+        os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     defaults:
       run: