AuthManager replaces the createaccount API; the new API is meant for generating UIs for humans (the fields and steps are not predictable and depend on what types of auth extensions are enabled). For browser tests, we would need a machine-oriented API though, something like the API version of user::newSystemUser. (Or maybe a session provider that identifies via some kind of shared secret and then just autocreates whatever username it is told to use?)
Example of errors caused by the lack of this: https://integration.wikimedia.org/ci/job/mwext-mw-selenium/6607/console
Given I am logged into the mobile website # features/step_definitions/common_steps.rb:28 The token parameter must be set (createnotoken) (MediawikiApi::ApiError) ./features/step_definitions/common_steps.rb:30:in `/^I am logged into the mobile website$/'