Add tests for FshToFhir API for more thorough coverage

[mint-thompson]

Transferred from internal JIRA issue originally created 2023-09-18.

The tests for the FshToFhir API should be built out more to test more of what it does. There may be other tests to add, but possible things to add include:

• testing what types (profiles/extensions/logicals/etc) are converted
• testing the full end to end function works (taking some FSH and ensuring the appropriate FHIR is exported)