Refactor header_spec.js

What does this MR do and why?

This MR refactors header_spec.js and replaces the use of then with async/await. No new tests were added or removed.

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.

Screenshots or screen recordings

There is no UI/UX changes

How to set up and validate locally

1. Test passes
2. Profit 💰

Related to #434947 (closed)