[QA] Testing thanks: How to do a browser test which involves 2 users

[Jon Robson]

Sometimes, a test requires two users. We currently have MEDIAWIKI_USER
but do we have another special account to act as another user?

I'm trying to write a test for the thanks feature:
https://gerrit.wikimedia.org/r/#/c/146628/

The step I'm stuck on is "the most recent topic is written by another
user", a step which involves creating some content by a user who is
not MEDIAWIKI_USER

How can I achieve this in a standard way? Are any other projects doing
something similar?