Fixes this warning when running test:

    UserWarning: Error validating request parameters against schema definition:
    RefResolutionError unknown url type: 'instance-input-schema.json'

jsonschema (the python library) support this kind of refs when using
version draft-06 but not when using draft-04.

json-language-features (from theia text editor) does not understands
file.json#/ references, it complains:

    $ref '/' in 'file://...' can not be resolved.(768)

but not using the trailing / works fine