This adds support for xUnit.NET 2 and improves the performance of building and testing. Building is improved by getting MSBuild to build everything in parallel, rather than invoking MSBuild for each project, and testing is improved because xUnit.NET 2 supports testing multiple assemblies in parallel. This means that the Arcanist unit test runner only needs to exec two processes for the whole operation, instead of two processes per assembly.
This also depends on D11688 landing in libphutil first.