Add test project and related infra where utests can be added for the VMR's build infrastructure

[MichaelSimons]

A new VMR test project is need to accommodate unit tests for the VMR's infrastructure. A good example of this the infrastructure for mutating the repo nuget.configs. This is critical code that would benefit from unit tests. Having this infra in place would help eliminate "excuses" for not writing tests for the VMR's infra.

[ghost]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichaelSimons]

This is being implemented as part of #4706.

[NikolaMilosavljevic]

Fixed with dotnet/sdk#44649