Refactor delayed_job_mixin_spec to use vue test utils

What does this MR do?

This refactors the delayed_job_mixin_spec.js file to use vue-test-utils instead of a full Vue instance for the specs

#320936 (closed)

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content