Consider parallel or full switch of CI to VSTS

[Oceanswave]

VSTS now has Linux and macOS build agents in preview. Consider switching over to VSTS. And advantage will be that the Windows test harness can start auto-reporting code coverage again as the enterprise bits required to do so are on VSTS

[Oceanswave]

Thanks to coverlet, code coverage is now auto-reported again, so this item becomes less pressing.

It would be nice, however, to have VSTS builds in addition to the existing appveyor/travisci for additional validation.