Draft: Fix: Spec for work item tree in Vue3 migration

What does this MR do and why?

Related issue #413401 (closed)

Fix spec for spec/frontend/work_items/components/work_item_links/work_item_tree_spec.js spec to help with the Vue 3 migration

The Error is Failed setting prop "children" on <work-item-children-wrapper-stub>

/cc @prajnamas @JeremyWuuuuu @qk44077907

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