Add a chutney job that runs on macOS, so that IPv6 chutney tests work
Travis Linux doesn't support IPv6, so we should add a macOS chutney job to Tor's CI.
If that's too slow, we can:
- add the chutney tests to the end of an existing macOS job
- change the chutney job to macOS
Remember: each build has a limit of 2 concurrent macOS jobs.
We can do this task after #29280 (moved) merges.