Some small Vue 3 spec fixes

What does this MR do and why?

Fixes a couple of Jest specs to work under 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.

Screenshots or screen recordings

n/a

How to set up and validate locally

These specs fail on master, but should pass here.

VUE_VERSION=3 yarn jest ee/spec/frontend/workspaces/user/components/workspace_variables_spec.js spec/frontend/diffs/stores/legacy_diffs/getters_spec.js