Add end-to-end tests

For each method from the public API, there should be one test case:

- Mock the HTTP request, returning data from a json file that was copyed from the official api docs. (For file reading, the existing `FileReaderUtil` can be used.)
- call the method that should be tested
- verify that the result equals the result returned from the relevant TestProvider

Start by listing the methods exposed by the public API and reviewing the existing TestProvider and FileReaderUtil. For each method, mock its HTTP request with JSON copied from the official API documentation, call the method, and compare the result with the relevant TestProvider result. Done means every public API method has one passing test case.

Written by the indexing model from the issue text.