Add common interface tests for minimum required functionality

We should have a description of the minimum required functionality that a Stan interface must implement.
This may include

• compiling a Stan model from a file
• sampling of the model
• optimization of the model
• repeatability of results given a fixed seed
• data I/O
• exporting generated samples

Additional:

• what visualizations are available

• posterior analysis (this should be further broken down)

• R-hat

• n_eff

• extractions

• samples +/- warmup +/- scrambling +/- structured extraction

• operations allowed with Stan model

• log prob +/- constrained

• gradients

• higher order derivatives

• variable name access

• variable dimensions access

• transforms to and from constrained form