Create functional tests for all Model Files scenarios

[rogancarr]

As laid out in #2498 , we need scenarios to cover the Model Files functionality we want fully supported in V1.

Definitely need for V1

• I can train a model and save it as a file. This model includes the learner as well as the transforms
• I can use a model file in a completely different process to make predictions
• I can easily figure out which NuGets (and versions) I need to score an ML.NET model
• I can export ML.NET models to ONNX (limited to the existing internal functionality)

May not need for now:

• I can save a model to text
• I can use newer versions of ML.NET with ML.NET model files of previous versions (for v1.x)
• I can use model files interchangeably between compatible versions of ML.NET and NimbusML
• I can move data between NimbusML and ML.NET (using IDV). Prepare with NimbusML and load with ML.NET