Add doc for setting up test users

What does this MR do and why?

Adds a doc for setting up test users. I'm adding it just to document the problem we ran into in https://gitlab.com/gitlab-org/quality/quality-engineering/team-tasks/-/issues/2089#note_1611660801.

We could add more to this document at a later date, or even better, shift away from testing on environments where we can't create users via the API.

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