Fix flaky pipeline spec

What does this MR do?

This fixes a spec that turned out to be flaky for me (reproducible locally and in CI). It seems like this has been around for a while and I'm not particularly sure why that hasn't shown up earlier a lot (perhaps just luck?).

The spec checks the order_by: 'user_id' parameter but has 3 pipelines (amongst 4 total) for the same user. Hence the specific order of pipelines by ORDER BY user_id DESC is not well defined for pipelines with the same user_id and this makes it a flaky spec.

We now just create 3 different users (one for each pipeline) - which I think is the original intent of the spec anyways.