Add dotnet msbuild integration tests for restore

[emgarten]

Add dotnet msbuild integration tests for restore

This should cover restoring solution files directly which is not covered by the nuget.exe tests since they parse the solution file first.

As part of this unit tests should be added for NuGet.Build.Tasks which passes in actual TaskItems. Currently this is tested at a lower level using the internal task abstraction, but it should also be tested at the higher level.