Authentication: Add tests that cover the service/wizards.

[pixlwave]

This can be taken in 2 parts:

• Test registration part of AuthenticationService and RegistrationWizard (ready now).
• Test login part of AuthenticationService and LoginWizard (pending implementation).

Probably the easiest way to test these would be to make an AuthenticationRestClient/AuthenticationHTTPClient protocols based on the MXRestClient+Async/MXHTTPClient+Async extensions, make mocks of both and allow AuthenticationService to be configured with the type of client it should create.

For reference: Android tests although they're from the view model and use mock service/wizards.