[Discover] Making source filters test run with fields API

[majagrubic]

Summary

Follow-up on:
#89837

Since fields API now supports source filtering, enabling a functional test to verify this behavior

Checklist

Delete any items that are not applicable to this PR.

- [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
- [ ] Documentation was added for features that require explanation or tutorials

• Unit or functional tests were updated or added to match the most common scenarios
  - [ ] Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
  - [ ] Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
  - [ ] If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the [docker list](https://github.com/elastic/kibana/blob/c29adfef29e921cc447d2a5ed06ac2047ceab552/src/dev/build/tasks/os_packages/docker_generator/resources/bin/kibana-docker)
  - [ ] This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
  - [ ] This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 9fbed9d

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[kertal]

Code LGTM, didn't test, but Jenkins did. One possible improvement, you could run this test with and without searchFieldsFromSource enabled then both cases would be covered