[React@18] Refactor overview.tsx test to rtl (#206805)

## Summary

Extracted from #206411
[[job]](https://buildkite.com/elastic/kibana-pull-request/builds/267344#019469ff-7fb9-4c5d-8569-2e445aab27be)
[[logs]](https://buildkite.com/organizations/elastic/pipelines/kibana-pull-request/builds/267344/jobs/019469ff-7fb9-4c5d-8569-2e445aab27be/artifacts/01946a1c-62fa-4d30-8863-1b40f8c0b924)
Jest Tests #9 / Overview renders correctly when there is no user data
view
This simplifies overview.tsx by refactoring to rtl and removing the
whole snapshot. The snapshot was not useful and the test is still making
sure that the intended component is still rendered. By removing enzyme,
the test now works properly for both react 17 and 18.