Provide an ability to configure primary resource for the CREATE tests

[projkov]

Original issue:

# Create test fails if the resource being created has an external reference that cannot be resolved

MedicationStatement create test (1.5.07) fails as it has a reference to an external Medication resource. 
What is the best way to get external references included in the server during a test run? In this failing test, 
the MedicationStatement resource is coming from the IG. The external Medication resource referenced is also in the IG.

Reference: aehrc/smart-health-checks-inferno#17

---

In this case, the core of the problem is the resource, which references another FHIR resource that does not exist on the server. This can be fixed by editing the resource.

However, there is no mechanism to move this resource to the top of the list. I think we should have the ability to designate specific resources as primary. For example, we could use an element in the config file that references a specific file containing the resource data.