azure-pipelines: test both 10.13 and 10.14

[scpeters]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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There is now a macOS-10.14 image at Azure pipelines:

• https://github.com/Microsoft/azure-pipelines-image-generation/blob/master/images/macos/macos-10.14-Readme.md

I'm not sure if we want to run our CI on both macOS platforms, but I thought I would show the syntax for doing so for brew itself, and in the script generated by dev-cmd/tap-new.

[MikeMcQuaid]

Neat, thanks. I reckon we just want to test on only Mojave here and tap-new (and similarly on the brew test-bot repository) until we're providing a bottle generation solution.

[scpeters]

Ok, I'll just open a separate PR for that.