MockAuthWebViewViewModel

To test AuthWebViewController without any actual provider.

1. It spends some time in the “preparing” state sometimes skipping it.
2. Then pretends that it got an endpoint (an inline page) with a link to a redirect URL. Tapping the link should cause the web view to intercept it and notify the view model via handleRedirect().
3. The latter is going to jump to a “finalizing” state spending some time there (possibly skipping it) and then jumping to “completed”. And of course it can randomly fail at any step.