Fix Work item unit tests for Vue 3 mode

What does this MR do and why?

This merge request fixes Work item unit tests for Vue 3 mode. The changes primarily focus on updating the test files to ensure compatibility with Vue 3 and improving the overall test coverage for the Work Items feature.

Specifically, this MR:

1. Adds data-testid attributes to various components in the work_item_time_tracking.vue file to improve test selectability.
2. Updates the work_item_labels_spec.js file to use a more accurate method of checking for scoped labels.
3. Modifies the todos_toggle_spec.js and work_item_drawer_spec.js files to use component stubs, which can help isolate component behavior in tests.
4. Significantly refactors the work_item_time_tracking_spec.js file to use shallowMount instead of mount, updates import statements, and modifies test selectors to use the newly added data-testid attributes.

MR acceptance checklist

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Screenshots or screen recordings

No visual changes

How to set up and validate locally

Run the following tests in Vue 3 mode:

VUE_VERSION=3 yarn jest ee/spec/frontend/work_items/components/work_item_labels_spec.js
VUE_VERSION=3 yarn jest spec/frontend/work_items/components/shared/todos_toggle_spec.js
VUE_VERSION=3 yarn jest spec/frontend/work_items/components/work_item_drawer_spec.js
VUE_VERSION=3 yarn jest spec/frontend/work_items/components/work_item_time_tracking_spec.js