MAGN-5402 Separate system test base into its own library.

This PR separates the base class for system tests into the SystemTestServices library. It then wires this library to a new sample library called SampleLibraryTests. The SampleLibraryTests library will then become the basis for a NuGet package DynamoVisualProgramming.Tests (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5404).

After these changes, our core UI tests run, and our Revit system tests run, both of which use this refactored base class.

PTAL:
- [ ] @lukechurch 

Requires Merge:

- [ ] Revit2015