Add Testing Tasks

[dansiegel]

Description

Unit testing Xamarin Application is a particular pain point as there is no good tooling for this. While developers can write unit tests for their netstandard libraries with a netcoreapp, this does not guarantee that the code written is in fact going to work as expected on Xamarin targets. Also when consuming libraries like Xamarin.Essentials which provide a bait and switch API you may simply get an Exception that prevents you altogether from testing code without a lot of abstraction.

Proposal

Take the work that has been done in the v1.1 branch of Xamarin Essentials to run the xunit device runners on Azure DevOps hosted pipelines and convert this into build tasks that will allow developers to easily run unit tests for Xamarin iOS, Xamarin Android and UWP as part of their pipeline. This task should really only need to point to the test project and know if which platform we're testing for, for basic usage. This should be able to then report to Azure Pipelines the results of our Unit Tests including test coverage.