Resolve "ee/spec/frontend/vue_shared/components/customizable_dashboard/customizable_dashboard_spec.js fails"

What does this MR do and why?

This MR fixes the master:broken by making sure that the loadCSSFile mock resolves successfully in the success test cases. The situation where loadCSSFile fails is already covered.

Screenshots or screen recordings

N/A

How to set up and validate locally

See Product Analytics dashboard widgets structure (!98613 - merged) but in this case, the tests passing should be sufficient 👍

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 #378311 (closed)