Fix spec violations in work_item_tree_spec.js

What does this MR do and why?

Resolves #413401 (closed)

Fixes spec violations in spec/frontend/work_items/components/work_item_links/work_item_tree_spec.js for vue3-migration.

Screenshots or screen recordings

No visible change

How to set up and validate locally

• Check pipeline or run yarn jest spec/frontend/work_items/components/work_item_links/work_item_tree_spec.js

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #413401 (closed)