Test: create new user to avoid 429s in spec

What does this MR do and why?

In the protected branch spec, we are testing that we can search using the drop down list. Over time, more tests have been added to this file, and now we are in a situation where we are doing so many searches using the same user, that some tests fail due 429 (throttling). This MR creates a new user per test. disables rate limiting in the spec.

Background: !135521 (comment 1631271586)

Master pipeline job showing the 429 error: https://gitlab.com/gitlab-org/gitlab/-/jobs/5449284085#L437

Job from this MR without 429 errors: https://gitlab.com/gitlab-org/gitlab/-/jobs/5473355019

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.