Add a test for User#ci_authorized_runners
## What does this MR do?
Add a test for User#ci_authorized_runners
## Are there points in the code the reviewer needs to double check?
Should we add `Project#add_user` so that it's similar to `Group#add_user`?
## Why was this MR needed?
I am going to make some changes to this method for !4093, but I couldn't find a corresponding test so I want to add it in case I would break anything.
## What are the relevant issue numbers?
This is a test for current behaviour, before implementing #3407
See merge request !4404
53bf9da4b4