Turn on MacOS tests on Cirrus (#19424)
Turn on MacOS tests on Cirrus (#19424)

Cirrus CI has macOS VMs, so this turns on an instance for tests so that we can evaluate if we can use Cirrus for all three platforms or not.
Hixie/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter