Use mock-apollo-client for testing in requirement_root_spec.js

Use mock-apollo-client to improve the spec quality of requirement_root_spec.js

This is a follow-up issue to implement the suggestion given during the review of !43583 (merged):

Nicolò Maria Mezzopera 🔵 @nmezzopera · 14 hours ago Maintainer

suggestion (non-blocking): we may want to consider using mock-apollo-client (https://docs.gitlab.com/ee/development/fe_guide/graphql.html#testing-with-mocked-apollo-client) for those tests, maybe as a followup?