Add tests for pagination

The following discussion from !79834 (merged) should be addressed:

• @dftian started a discussion: (+2 comments)

  @stanhu Would you be able to do the backend maintainer review for this one? 🙇

How to verify

Check that ee/spec/frontend/security_dashboard/components/shared/vulnerability_report/vulnerability_list_graphql_spec.js has tests for the pagination feature, under describe('pagination').