Fix spec violations for file_finder component

What does this MR do and why?

The spec file spec/frontend/vue_shared/components/file_finder/index_spec.js contains technical debt and/or antipatterns that need to be fixed. This is a prerequisite for updating our codebase to Vue3.

Screenshots or screen recordings

N/A

How to set up and validate locally

Run the spec:

npm run jest -- spec/frontend/vue_shared/components/file_finder/index_spec.js

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 #391110 (closed)