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