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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Yes, this is why we need separate target: to specify separate testsettings file. For more context see https://github.com/Azure/azure-sdk-tools/pull/2925/files#diff-f9a5915fb1c67f17a832b7084b5d5f27R29