Fix work item tests to pass in Vue 3

What does this MR do and why?

This MR fixes a couple of unit tests that were passing in Vue 2 but failing in Vue 3

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Run the following tests in Vue 3 mode:

VUE_VERSION=3 yarn jest spec/frontend/work_items/components/work_item_prefetch_spec.js
VUE_VERSION=3 yarn jest spec/frontend/work_items/components/work_item_detail_modal_spec.js
VUE_VERSION=3 yarn jest spec/frontend/boards/components/board_drawer_wrapper_spec.js