Implement fhirpath-js test runner

[johngrimes]

This will run all tests in the fhirpath-js test suite.

We will need to be able to execute expressions against arbitrary data structures that are not strictly FHIR.

We will also need a way to exclude tests that are not passing, and describe the reason for exclusion with an optional reference to a GitHub issue.

[johngrimes]

@piotrszul Great work in getting this test runner working!

As discussed, the next step is to go through the results and create tickets to resolve. We can work out which ones we want to fix, and which ones we want to exclude from the implementation for now.