Refactored Karma spec to Jest for todo_spec.js

What does this MR do?

Migrates spec/javascripts/sidebar/todo_spec.js to Jest & vue-test-utils

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Closes #31306 (closed)