Refactor test to handle response with "files" key

[tripledoublev]

list_files returns an object containing the array under the files key

this PR refactors the test to access the array via the proper key.

here is an example response from the test:
{"files": Array [Object {"hash": String("vrcirtfznqnjwuzxexbdcgxqarg6i3gjhkmdniyj6pyo5ii7l25a"), "is_downloaded": Bool(true), "name": String("example.txt")}]}