Global Search Refactor - Focus search shortcut tests

What does this MR do and why?

This addresses a regression found in Staging: #345745 (comment 755156321)
Work towards #297396 (closed)

This change is behind a feature flag (:new_header_search)
Feature Flag Rollout Issue: #339348 (closed)

This change re-adds the missing feature to focus the Header Search when pressing the focus search shortcut key. This key can be set in a user's preferences but by default the key is (s or /)

Thank you @dgruzd for spotting this!

Screenshots or screen recordings

No visual change

How to set up and validate locally

1. Fetch and checkout this branch
2. Activate the feature flag (:new_header_search)
3. Go to your GitLab homepage (ex http://127.0.0.1:3000) and ensure no inputs are currently focused
4. Hit the "S" key and ensure it focuses the search field at the top right of the header bar
5. Unfocus the header search . Hit the "/" key and ensure it focuses the search field at the top right of the header bar.

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.