Extend end-to-end tests for new ES admin UI

Summary

This is a follow-up to !17230 (closed) / !18254 (merged) / !20421 (merged).

The changes are tested through lower-level specs, and some high-level RSpec feature specs. To increase the end-to-end test coverage, it's not clear yet if we want to extend the RSpec feature specs, or implement this as part of the QA test suite.

This should be done once we're getting ready to release the new ES admin UI.

Improvements

Involved components

• ee/spec/features/admin/admin_elasticsearch_spec.rb
• qa/qa/ee/page/admin/elasticsearch
• qa/qa/ee/resource/settings/elasticsearch.rb
• qa/qa/ee/scenario/test/integration/elasticsearch.rb
• qa/qa/specs/features/ee/browser_ui/enablement/elasticsearch/

In https://gitlab.com/gitlab-org/gitlab-qa:

• lib/gitlab/qa/component/elasticsearch.rb
• lib/gitlab/qa/scenario/test/integration/elasticsearch.rb