Add snapshot testing

[dionmcm]

Testing the AMT transformation to FHIR is hard, one cheap way to do this is snapshot testing, where the resources can be generated against a known data set and stored. They can then be regenerated and compared at build time to determine if changes have occurred.

Jest provides a good way to do this, something similar in concept is needed https://facebook.github.io/jest/docs/en/snapshot-testing.html

Note this isn't comprehensive testing, but a cheap step forward once the transform attains decent stability (i.e. the target stops moving so much!)