Update tests in dismissal_comment_modal_footer_spec.js

What does this MR do and why?

Update tests in file dismissal_comment_modal_footer_spec.js, removed toBeTruthy matchers and replaced with more specific matchers.

For issue #369723 (closed)

Screenshots or screen recordings

n/a

How to set up and validate locally

yarn jest ee/spec/frontend/vue_shared/security_reports/dismissal_comment_modal_footer_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.