Add domain expertise for backend testing performance

Context

In gitlab-org/gitlab#387973 (closed), we are working on assigning known slow backend tests to the specific teams/groups. We are going to link https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#test-speed for guidance on how to improve slow tests in a self-service manner.

Why is this change being made?

In another MR, I'd like to to include a line in the docs saying that people can reach out to people with the backend_testing_performance domain expertise in case they are stuck on slow tests refactorings.

This MR introduces a new backend_testing_performance domain expertise to help the engineers that would need some guidance on how to troubleshoot/fix slow tests.

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