Fix rate limit spec for parallel compatibility

What does this MR do and why?

While running tests in parallel using the same instance of GitLab, when we modify the rate limits at the application level, all other tests in another thread can get blocked and fail with `TooManyRequests`. This introduces the scoping at the user level, restricting the throttling to the user created in this spec.

This work is part of Enable E2E test parallelization on multiple CPU cores

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.