Add tests for DscBuild

[dlwyatt]

There are currently very few tests for Invoke-DscBuild and its various pieces. Part of this is due to how the module currently works, where each function is called with basically no parameters, and information is passed around via script-scope variables instead (in both directions).

It'll be easier to test, and for people to understand the code, if we refactor these so all function input is via parameters and all function output is via the output stream.

[ArtWDrahn]

This one has been on my list for a while, been trying to prove out my skills with the other modules before trying to tackle it. I am completely in favor of refactoring this to be more testable.