[wix-devs] No more traversing

[Bob Arnson]

Apparently, it's a long-standing to-be-implemented feature to be able to run .NET Core tests from desktop MSBuild.

https://github.com/microsoft/vstest/issues/326

We can't use Core MSBuild because it pretends C++ doesn't exist. So we're stuck - we can't use traversal projects to do the normal restore/build/test/pack dance.

I'll look around the tree to see if there are any other trees that could be made into traversal projects, but it looks like most of the WiX v4 build is going to be single-proc. Yay.