[model-test] referencing external schema

We are currently developing json schemas to help test for implementations of
SpecificResource features of the data model.  I've uploaded 4 of the schemas
currently being developed to
https://github.com/w3c/web-annotation-tests/tree/master/SpecificResource
(these are first drafts, not quite ready, still being debugged and refined).
All 4 schemas share a definitions section containing 2 sub-schemas (appended
below).  These are easy to reference internally where needed, but it would
be even more convenient to reference them as external schemas.  This should
be possible using $ref, but I've had difficulties getting this approach to
work with my local ajv instance, especially with regard to relative file
locations.
 
Is anyone familiar enough with how $ref works within the Web Platform
Testing framework's implementation of ajv to advise?
 
Below is the current way the sub-schemas are embedded in each primary
schema. Obviously this works, but it means repeating the same sub-schemas in
multiple files, which is obviously less efficient than if these sub-schemas
could be referenced as individual files. 
 
Thanks,
Tim Cole
 
{
    "definitions": {
        "SpecificResource": {
            "type": "object",
            "properties": {
                "selector": {},
                "state": {},
                "styleClass": {},
                "renderedVia": {},
                "scope": {},
                "purpose": {}
            },
            "anyOf": [
                {"required": ["selector"]},
                {"required": ["state"]},
                {"required": ["styleClass"]},
                {"required": ["renderedVia"]},
                {"required": ["scope"]},
                {
                    "required": ["purpose"],
                    "not": {"required": ["value"]}
                }
            ]
        },
        "SpecificResType" : {
            "type": "object",
            "properties": {
                "type": {
                    "oneOf": [
                        {"type": "string",
                         "enum": ["SpecificResource"]},
                        {"type": "array",
                         "items": [{ "type": "string",
                                    "enum": ["SpecificResource"] }]
                        } ]
                }
            },
           "required": ["type"]
       }
    }
}