Add MacOS build testing

[andrewc12]

The current appveyor config for building and testing on macos.
Signed-off-by: Andrew Innes andrew.c12@gmail.com

Motivation and Context

We're unable to run zfs tests on the macos github images, but we still need to be able to automatically run tests.

Description

By adding the config files required to build and test zfs on macos on appveyor, any forked repo with appveyor integration enabled will automatically run the zfs tests on macos.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[andrewc12]

⚠️ This PR needs PR #12110 to be merged first ⚠️

[andrewc12]

It's possible to move appveyor.yaml to a cleaner location but that requires setting up appveyor in a specific way.
"YAML file alternative naming"
https://www.appveyor.com/docs/build-configuration/#:~:text=Pull%20request%E2%80%9D%20events-,YAML%20file%20alternative%20naming,-AppVeyor%20supports%20dot