Fix spec Vue3 spec violations in blob_header_spec

What does this MR do and why?

Fix spec violations in spec/frontend/blob/components/blob_header_spec.js

Screenshots or screen recordings

none

How to set up and validate locally

green pipeline

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.

Related to #391182 (closed)