DSC Extension: Add unit tests for cmdlets components and enable them in the azure jankens build process [2]

This PR includes:
1) Refactoring to improve testability
2) New Unit tests for -ConfigurationArguments processing
3) Make all DSC extension UTs and BVTs checkin tests
4) New test assembly for extensions UTs and BVTs.
5) Bug fix for DSC extension DSCResource resolution logic
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As discussed offline, please remove parts that are not used now.