User Testing – task definition problem for "Login / Forgot password"

The iOs app that we are building is going to be prototype tested soon in the usability lab and one of the flows we want to test is the forgot password flow to see if users can easily find and use the flow which in the companies case is a bit more complicated then for most other systems. The questions arises for me how to define and write the user task for this test, is it necessary to inform the user that he/she has forgotten the password and that he/she should use the app to retrieve the username/password or would it be more realistic to brief the user that he/she hasnt used the service for over a year and that probably the username/password is xx/yy, so to give fuzzy information and to have the user fail to login and figure out autonomously how to advance from there.

The dilemma is, if the user is informed explicitly about the lack of password knowledge he is too determined to find the help/retrieve button. On the other hand the user might be confused in the test situation if the provided password is not accepted.

What are your thoughts on this?