Add a test suite for user-related methods

[AngeloStavrow]

Add unit tests for the public API related to users in WFClient:

• login(username:​password:​completion:​)
• logout(token:​completion:​)
• get​User​Data(token:​completion:​)
• get​User​Collections(token:​completion:

See the developer API docs for useful responses that can be mocked, as in #38.