JSON frontend ignores scenario constraints

[jmafoster1]

When processing tests, the JSON frontend ignores any constraints passed into the modelling scenario. I believe this is because the Scenario object isn't passed into the JsonUtility, so it doesn't know about the constraints. I'm not sure how it's even generating concrete test cases without the modelling scenario, but we definitely need at least the constraints. My personal feeling is that we should be passing in the Scenario object instead of doing json_utility.set_variables(inputs, outputs, []).