Add support for expression maps to YAML test runner

[johngrimes]

There is an expression -> map feature used within the FHIRPath.js tests, looks something like this:

tests:
  - desc: Evaluating expression for a part of a resource
    inputfile: questionnaire-part-example.json
    model: 'r4'
    expression: {base: 'QuestionnaireResponse.item', expression: 'answer.value = 2 year'}
    result:
      - true

We don't currently support it and have to exclude this test, it would be good if we could pass it.